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linux-swiotlb-for-linux-4.14.113.patch
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linux-swiotlb-for-linux-4.14.113.patch
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From f931fac6184ffd46e9223d979f1997199f0d29e1 Mon Sep 17 00:00:00 2001
From: Dongli Zhang <[email protected]>
Date: Mon, 27 May 2019 11:04:26 +0800
Subject: [PATCH 1/1] linux-swiotlb-for-linux-4.14.113
swiotlb linux-4.4.113
Signed-off-by: Dongli Zhang <[email protected]>
---
arch/x86/include/asm/dma-mapping.h | 9 +
arch/x86/include/asm/iommu_table.h | 10 +
arch/x86/kernel/pci-dma.c | 6 +
arch/x86/kernel/pci-swiotlb.c | 124 +++++++
arch/x86/xen/pci-swiotlb-xen.c | 62 ++++
drivers/xen/swiotlb-xen.c | 524 +++++++++++++++++++++++++++
include/linux/device.h | 2 +
include/linux/dma-direction.h | 8 +
include/linux/dma-mapping.h | 64 ++++
include/linux/swiotlb.h | 39 ++
lib/swiotlb.c | 706 ++++++++++++++++++++++++++++++++++++-
11 files changed, 1551 insertions(+), 3 deletions(-)
diff --git a/arch/x86/include/asm/dma-mapping.h b/arch/x86/include/asm/dma-mapping.h
index 69f16f0..6a03b19 100644
--- a/arch/x86/include/asm/dma-mapping.h
+++ b/arch/x86/include/asm/dma-mapping.h
@@ -50,6 +50,11 @@ extern phys_addr_t dma_to_phys(struct device *dev, dma_addr_t daddr);
static inline bool dma_capable(struct device *dev, dma_addr_t addr, size_t size)
{
+ /*
+ * dma_mask的类型:
+ *
+ * u64 *dma_mask;
+ */
if (!dev->dma_mask)
return 0;
@@ -74,6 +79,10 @@ dma_cache_sync(struct device *dev, void *vaddr, size_t size,
flush_write_buffers();
}
+/*
+ * 如果dev->coherent_dma_mask已经有了就返回dev->coherent_dma_mask
+ * 否则根据gfp决定是DMA_BIT_MASK(24)还是DMA_BIT_MASK(32)
+ */
static inline unsigned long dma_alloc_coherent_mask(struct device *dev,
gfp_t gfp)
{
diff --git a/arch/x86/include/asm/iommu_table.h b/arch/x86/include/asm/iommu_table.h
index 1fb3fd1..1a5c46c 100644
--- a/arch/x86/include/asm/iommu_table.h
+++ b/arch/x86/include/asm/iommu_table.h
@@ -87,9 +87,19 @@ struct iommu_table_entry {
* Both variants will still call the 'init' and 'late_init' functions if
* they are set.
*/
+/*
+ * used by:
+ * - arch/x86/kernel/pci-swiotlb.c|128| <<global>> IOMMU_INIT_FINISH(pci_swiotlb_detect_override,
+ * - arch/x86/xen/pci-swiotlb-xen.c|130| <<global>> IOMMU_INIT_FINISH(pci_xen_swiotlb_detect,
+ * - drivers/iommu/amd_iommu_init.c|2932| <<global>> IOMMU_INIT_FINISH(amd_iommu_detect,
+ */
#define IOMMU_INIT_FINISH(_detect, _depend, _init, _late_init) \
__IOMMU_INIT(_detect, _depend, _init, _late_init, 1)
+/*
+ * used by only:
+ * - arch/x86/kernel/pci-swiotlb.c|172| <<global>> IOMMU_INIT(pci_swiotlb_detect_4gb,
+ */
#define IOMMU_INIT(_detect, _depend, _init, _late_init) \
__IOMMU_INIT(_detect, _depend, _init, _late_init, 0)
diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
index 599d746..c43814c 100644
--- a/arch/x86/kernel/pci-dma.c
+++ b/arch/x86/kernel/pci-dma.c
@@ -76,6 +76,12 @@ void __init pci_iommu_alloc(void)
}
}
}
+/*
+ * used by:
+ * - arch/x86/kernel/pci-nommu.c|100| <<global>> .alloc = dma_generic_alloc_coherent,
+ * - arch/x86/kernel/amd_gart_64.c|503| <<gart_alloc_coherent>> return dma_generic_alloc_coherent(dev, size, dma_addr, flag,
+ * - arch/x86/kernel/pci-swiotlb.c|51| <<x86_swiotlb_alloc_coherent>> vaddr = dma_generic_alloc_coherent(hwdev, size, dma_handle, flags,
+ */
void *dma_generic_alloc_coherent(struct device *dev, size_t size,
dma_addr_t *dma_addr, gfp_t flag,
unsigned long attrs)
diff --git a/arch/x86/kernel/pci-swiotlb.c b/arch/x86/kernel/pci-swiotlb.c
index 53bd05e..8b88cf5 100644
--- a/arch/x86/kernel/pci-swiotlb.c
+++ b/arch/x86/kernel/pci-swiotlb.c
@@ -15,8 +15,26 @@
#include <asm/xen/swiotlb-xen.h>
#include <asm/iommu_table.h>
+/*
+ * 在以下修改swiotlb:
+ * - arch/x86/kernel/amd_gart_64.c|855| <<gart_iommu_init>> swiotlb = 0;
+ * - arch/x86/kernel/pci-dma.c|200| <<iommu_setup>> swiotlb = 1;
+ * - arch/x86/kernel/pci-swiotlb.c|91| <<pci_swiotlb_detect_override>> swiotlb = 1;
+ * - arch/x86/kernel/pci-swiotlb.c|109| <<pci_swiotlb_detect_4gb>> swiotlb = 1;
+ * - arch/x86/kernel/pci-swiotlb.c|118| <<pci_swiotlb_detect_4gb>> swiotlb = 1;
+ * - arch/x86/kernel/tboot.c|536| <<tboot_force_iommu>> swiotlb = 0;
+ * - arch/x86/xen/pci-swiotlb-xen.c|43| <<pci_xen_swiotlb_detect>> swiotlb = 0;
+ * - drivers/iommu/amd_iommu.c|2803| <<amd_iommu_init_dma_ops>> swiotlb = (iommu_pass_through || sme_me_mask) ? 1 : 0;
+ * - drivers/iommu/intel-iommu.c|4823| <<intel_iommu_init>> swiotlb = 0;
+ */
int swiotlb __read_mostly;
+/*
+ * called by:
+ * - include/linux/dma-mapping.h|512| <<dma_alloc_attrs>> cpu_addr = ops->alloc(dev, size, dma_handle, flag, attrs);
+ *
+ * struct dma_map_ops swiotlb_dma_ops.alloc = x86_swiotlb_alloc_coherent()
+ */
void *x86_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
dma_addr_t *dma_handle, gfp_t flags,
unsigned long attrs)
@@ -30,6 +48,12 @@ void *x86_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
*/
flags |= __GFP_NOWARN;
+ /*
+ * 在以下使用dma_generic_alloc_coherent():
+ * - arch/x86/kernel/pci-nommu.c|100| <<global>> .alloc = dma_generic_alloc_coherent,
+ * - arch/x86/kernel/amd_gart_64.c|503| <<gart_alloc_coherent>> return dma_generic_alloc_coherent(dev, size, dma_addr, flag,
+ * - arch/x86/kernel/pci-swiotlb.c|51| <<x86_swiotlb_alloc_coherent>> vaddr = dma_generic_alloc_coherent(hwdev, size, dma_handle, flags,
+ */
vaddr = dma_generic_alloc_coherent(hwdev, size, dma_handle, flags,
attrs);
if (vaddr)
@@ -38,6 +62,12 @@ void *x86_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
return swiotlb_alloc_coherent(hwdev, size, dma_handle, flags);
}
+/*
+ * called by:
+ * - include/linux/dma-mapping.h|533| <<dma_free_attrs>> ops->free(dev, size, cpu_addr, dma_handle, attrs);
+ *
+ * struct dma_map_ops swiotlb_dma_ops.free = x86_swiotlb_free_coherent()
+ */
void x86_swiotlb_free_coherent(struct device *dev, size_t size,
void *vaddr, dma_addr_t dma_addr,
unsigned long attrs)
@@ -48,6 +78,20 @@ void x86_swiotlb_free_coherent(struct device *dev, size_t size,
dma_generic_free_coherent(dev, size, vaddr, dma_addr, attrs);
}
+/*
+ * e1000e一段代码例子:
+ * 693 buffer_info->dma = dma_map_single(&pdev->dev, skb->data,
+ * 694 adapter->rx_buffer_len,
+ * 695 DMA_FROM_DEVICE);
+ * 696 if (dma_mapping_error(&pdev->dev, buffer_info->dma)) {
+ * 697 dev_err(&pdev->dev, "Rx DMA map failed\n");
+ * 698 adapter->rx_dma_failed++;
+ * 699 break;
+ * 700 }
+ *
+ * used by:
+ * - arch/x86/kernel/pci-swiotlb.c|127| <<pci_swiotlb_init>> dma_ops = &swiotlb_dma_ops;
+ */
static const struct dma_map_ops swiotlb_dma_ops = {
.mapping_error = swiotlb_dma_mapping_error,
.alloc = x86_swiotlb_alloc_coherent,
@@ -69,13 +113,57 @@ static const struct dma_map_ops swiotlb_dma_ops = {
* This returns non-zero if we are forced to use swiotlb (by the boot
* option).
*/
+/*
+ * [0] pci_swiotlb_detect_override
+ * [0] pci_iommu_alloc
+ * [0] mem_init
+ * [0] start_kernel
+ * [0] secondary_startup_64
+ *
+ * 被IOMMU_INIT()和IOMMU_INIT_FINISH()使用
+ */
int __init pci_swiotlb_detect_override(void)
{
+ /*
+ * 在以下修改swiotlb:
+ * - arch/x86/kernel/amd_gart_64.c|855| <<gart_iommu_init>> swiotlb = 0;
+ * - arch/x86/kernel/pci-dma.c|200| <<iommu_setup>> swiotlb = 1;
+ * - arch/x86/kernel/pci-swiotlb.c|91| <<pci_swiotlb_detect_override>> swiotlb = 1;
+ * - arch/x86/kernel/pci-swiotlb.c|109| <<pci_swiotlb_detect_4gb>> swiotlb = 1;
+ * - arch/x86/kernel/pci-swiotlb.c|118| <<pci_swiotlb_detect_4gb>> swiotlb = 1;
+ * - arch/x86/kernel/tboot.c|536| <<tboot_force_iommu>> swiotlb = 0;
+ * - arch/x86/xen/pci-swiotlb-xen.c|43| <<pci_xen_swiotlb_detect>> swiotlb = 0;
+ * - drivers/iommu/amd_iommu.c|2803| <<amd_iommu_init_dma_ops>> swiotlb = (iommu_pass_through || sme_me_mask) ? 1 : 0;
+ * - drivers/iommu/intel-iommu.c|4823| <<intel_iommu_init>> swiotlb = 0;
+ */
if (swiotlb_force == SWIOTLB_FORCE)
swiotlb = 1;
return swiotlb;
}
+/*
+ * A more sophisticated version of IOMMU_INIT. This variant requires:
+ * a). A detection routine function.
+ * b). The name of the detection routine we depend on to get called
+ * before us.
+ * c). The init routine which gets called if the detection routine
+ * returns a positive value from the pci_iommu_alloc. This means
+ * no presence of a memory allocator.
+ * d). Similar to the 'init', except that this gets called from pci_iommu_init
+ * where we do have a memory allocator.
+ *
+ * The standard IOMMU_INIT differs from the IOMMU_INIT_FINISH variant
+ * in that the former will continue detecting other IOMMUs in the call
+ * list after the detection routine returns a positive number, while the
+ * latter will stop the execution chain upon first successful detection.
+ * Both variants will still call the 'init' and 'late_init' functions if
+ * they are set.
+ *
+ * used by:
+ * - arch/x86/kernel/pci-swiotlb.c|128| <<global>> IOMMU_INIT_FINISH(pci_swiotlb_detect_override,
+ * - arch/x86/xen/pci-swiotlb-xen.c|130| <<global>> IOMMU_INIT_FINISH(pci_xen_swiotlb_detect,
+ * - drivers/iommu/amd_iommu_init.c|2932| <<global>> IOMMU_INIT_FINISH(amd_iommu_detect,
+ */
IOMMU_INIT_FINISH(pci_swiotlb_detect_override,
pci_xen_swiotlb_detect,
pci_swiotlb_init,
@@ -85,10 +173,27 @@ IOMMU_INIT_FINISH(pci_swiotlb_detect_override,
* If 4GB or more detected (and iommu=off not set) or if SME is active
* then set swiotlb to 1 and return 1.
*/
+/*
+ * [0] pci_swiotlb_detect_4gb
+ * [0] pci_iommu_alloc
+ * [0] mem_init
+ * [0] start_kernel
+ * [0] secondary_startup_64
+ *
+ * 在kvm上此时swiotlb是0
+ */
int __init pci_swiotlb_detect_4gb(void)
{
/* don't initialize swiotlb if iommu=off (no_iommu=1) */
#ifdef CONFIG_X86_64
+ /*
+ * 在KVM的guest上:
+ * no_iommu=0, max_possible_pfn=1542144, MAX_DMA32_PFN=1048576
+ * 所以这里swiotlb设置为1
+ *
+ * pci_swiotlb_init()也调用了
+ * 否则应该不会调用pci_swiotlb_init()
+ */
if (!no_iommu && max_possible_pfn > MAX_DMA32_PFN)
swiotlb = 1;
#endif
@@ -103,11 +208,22 @@ int __init pci_swiotlb_detect_4gb(void)
return swiotlb;
}
+/*
+ * used by only:
+ * - arch/x86/kernel/pci-swiotlb.c|172| <<global>> IOMMU_INIT(pci_swiotlb_detect_4gb,
+ */
IOMMU_INIT(pci_swiotlb_detect_4gb,
pci_swiotlb_detect_override,
pci_swiotlb_init,
pci_swiotlb_late_init);
+/*
+ * [0] pci_swiotlb_init
+ * [0] pci_iommu_alloc
+ * [0] mem_init
+ * [0] start_kernel
+ * [0] secondary_startup_64
+ */
void __init pci_swiotlb_init(void)
{
if (swiotlb) {
@@ -116,6 +232,14 @@ void __init pci_swiotlb_init(void)
}
}
+/*
+ * [0] pci_swiotlb_late_init
+ * [0] pci_iommu_init
+ * [0] do_one_initcall
+ * [0] kernel_init_freeable
+ * [0] kernel_init
+ * [0] ret_from_fork
+ */
void __init pci_swiotlb_late_init(void)
{
/* An IOMMU turned us off. */
diff --git a/arch/x86/xen/pci-swiotlb-xen.c b/arch/x86/xen/pci-swiotlb-xen.c
index 37c6056..0b339e1 100644
--- a/arch/x86/xen/pci-swiotlb-xen.c
+++ b/arch/x86/xen/pci-swiotlb-xen.c
@@ -16,6 +16,12 @@
#endif
#include <linux/export.h>
+/*
+ * 只在以下修改xen_swiotlb:
+ * - arch/x86/xen/pci-swiotlb-xen.c|38| <<pci_xen_swiotlb_detect>> xen_swiotlb = 1;
+ *
+ * 在xen的测试机上是1
+ */
int xen_swiotlb __read_mostly;
/*
@@ -34,12 +40,28 @@ int __init pci_xen_swiotlb_detect(void)
* activate this IOMMU. If running as PV privileged, activate it
* irregardless.
*/
+ /*
+ * 在以下修改swiotlb:
+ * - arch/x86/kernel/amd_gart_64.c|855| <<gart_iommu_init>> swiotlb = 0;
+ * - arch/x86/kernel/pci-dma.c|200| <<iommu_setup>> swiotlb = 1;
+ * - arch/x86/kernel/pci-swiotlb.c|91| <<pci_swiotlb_detect_override>> swiotlb = 1;
+ * - arch/x86/kernel/pci-swiotlb.c|109| <<pci_swiotlb_detect_4gb>> swiotlb = 1;
+ * - arch/x86/kernel/pci-swiotlb.c|118| <<pci_swiotlb_detect_4gb>> swiotlb = 1;
+ * - arch/x86/kernel/tboot.c|536| <<tboot_force_iommu>> swiotlb = 0;
+ * - arch/x86/xen/pci-swiotlb-xen.c|43| <<pci_xen_swiotlb_detect>> swiotlb = 0;
+ * - drivers/iommu/amd_iommu.c|2803| <<amd_iommu_init_dma_ops>> swiotlb = (iommu_pass_through || sme_me_mask) ? 1 : 0;
+ * - drivers/iommu/intel-iommu.c|4823| <<intel_iommu_init>> swiotlb = 0;
+ *
+ * 这里是唯一可能修改xen_swiotlb的地方:
+ * - arch/x86/xen/pci-swiotlb-xen.c|38| <<pci_xen_swiotlb_detect>> xen_swiotlb = 1;
+ */
if (xen_initial_domain() || swiotlb || swiotlb_force == SWIOTLB_FORCE)
xen_swiotlb = 1;
/* If we are running under Xen, we MUST disable the native SWIOTLB.
* Don't worry about swiotlb_force flag activating the native, as
* the 'swiotlb' flag is the only one turning it on. */
+ /* 在xen上是0 */
swiotlb = 0;
#ifdef CONFIG_X86_64
@@ -54,9 +76,17 @@ int __init pci_xen_swiotlb_detect(void)
return xen_swiotlb;
}
+/*
+ * 在xen测试机上调用了
+ */
void __init pci_xen_swiotlb_init(void)
{
if (xen_swiotlb) {
+ /*
+ * xen_swiotlb_init()在以下调用:
+ * - arch/x86/xen/pci-swiotlb-xen.c|60| <<pci_xen_swiotlb_init>> xen_swiotlb_init(1, true );
+ * - arch/x86/xen/pci-swiotlb-xen.c|77| <<pci_xen_swiotlb_init_late>> rc = xen_swiotlb_init(1, false );
+ */
xen_swiotlb_init(1, true /* early */);
dma_ops = &xen_swiotlb_dma_ops;
@@ -67,6 +97,10 @@ void __init pci_xen_swiotlb_init(void)
}
}
+/*
+ * called by:
+ * - drivers/pci/xen-pcifront.c|703| <<pcifront_connect_and_init_dma>> err = pci_xen_swiotlb_init_late();
+ */
int pci_xen_swiotlb_init_late(void)
{
int rc;
@@ -78,6 +112,11 @@ int pci_xen_swiotlb_init_late(void)
if (rc)
return rc;
+ /*
+ * xen_swiotlb_dma_ops在以下使用:
+ * - arch/x86/xen/pci-swiotlb-xen.c|61| <<pci_xen_swiotlb_init>> dma_ops = &xen_swiotlb_dma_ops;
+ * - arch/x86/xen/pci-swiotlb-xen.c|81| <<pci_xen_swiotlb_init_late>> dma_ops = &xen_swiotlb_dma_ops;
+ */
dma_ops = &xen_swiotlb_dma_ops;
#ifdef CONFIG_PCI
/* Make sure ACS will be enabled */
@@ -88,6 +127,29 @@ int pci_xen_swiotlb_init_late(void)
}
EXPORT_SYMBOL_GPL(pci_xen_swiotlb_init_late);
+/*
+ * A more sophisticated version of IOMMU_INIT. This variant requires:
+ * a). A detection routine function.
+ * b). The name of the detection routine we depend on to get called
+ * before us.
+ * c). The init routine which gets called if the detection routine
+ * returns a positive value from the pci_iommu_alloc. This means
+ * no presence of a memory allocator.
+ * d). Similar to the 'init', except that this gets called from pci_iommu_init
+ * where we do have a memory allocator.
+ *
+ * The standard IOMMU_INIT differs from the IOMMU_INIT_FINISH variant
+ * in that the former will continue detecting other IOMMUs in the call
+ * list after the detection routine returns a positive number, while the
+ * latter will stop the execution chain upon first successful detection.
+ * Both variants will still call the 'init' and 'late_init' functions if
+ * they are set.
+ *
+ * used by:
+ * - arch/x86/kernel/pci-swiotlb.c|128| <<global>> IOMMU_INIT_FINISH(pci_swiotlb_detect_override,
+ * - arch/x86/xen/pci-swiotlb-xen.c|130| <<global>> IOMMU_INIT_FINISH(pci_xen_swiotlb_detect,
+ * - drivers/iommu/amd_iommu_init.c|2932| <<global>> IOMMU_INIT_FINISH(amd_iommu_detect,
+ */
IOMMU_INIT_FINISH(pci_xen_swiotlb_detect,
NULL,
pci_xen_swiotlb_init,
diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
index 95dbee8..f72987e 100644
--- a/drivers/xen/swiotlb-xen.c
+++ b/drivers/xen/swiotlb-xen.c
@@ -33,6 +33,14 @@
*
*/
+/*
+ * iommu (iommu_setup) is configured at arch/x86/kernel/pci-dma.c
+ *
+ * iommu=soft 似乎在所有操作系统上是default (swiotlb = 1)
+ *
+ * 在xen上swiotlb = 0
+ */
+
#define pr_fmt(fmt) "xen:" KBUILD_MODNAME ": " fmt
#include <linux/bootmem.h>
@@ -53,6 +61,12 @@
* API.
*/
+/*
+ * 忽略吧 和x86无关
+ *
+ * 对于x86 如果dev->coherent_dma_mask已经有了就返回dev->coherent_dma_mask
+ * 否则根据gfp决定是DMA_BIT_MASK(24)还是DMA_BIT_MASK(32)
+ */
#ifndef CONFIG_X86
static unsigned long dma_alloc_coherent_mask(struct device *dev,
gfp_t gfp)
@@ -69,12 +83,65 @@ static unsigned long dma_alloc_coherent_mask(struct device *dev,
#define XEN_SWIOTLB_ERROR_CODE (~(dma_addr_t)0x0)
+/*
+ * xen_io_tlb_start在以下使用:
+ * - drivers/xen/swiotlb-xen.c|193| <<is_xen_swiotlb_buffer>> return paddr >= virt_to_phys(xen_io_tlb_start) &&
+ * - drivers/xen/swiotlb-xen.c|378| <<xen_swiotlb_init>> xen_io_tlb_start = alloc_bootmem_pages(PAGE_ALIGN(bytes));
+ * - drivers/xen/swiotlb-xen.c|383| <<xen_swiotlb_init>> xen_io_tlb_start = (void *)xen_get_swiotlb_free_pages(order);
+ * - drivers/xen/swiotlb-xen.c|384| <<xen_swiotlb_init>> if (xen_io_tlb_start)
+ * - drivers/xen/swiotlb-xen.c|395| <<xen_swiotlb_init>> if (!xen_io_tlb_start) {
+ * - drivers/xen/swiotlb-xen.c|407| <<xen_swiotlb_init>> xen_io_tlb_end = xen_io_tlb_start + bytes;
+ * - drivers/xen/swiotlb-xen.c|412| <<xen_swiotlb_init>> rc = xen_swiotlb_fixup(xen_io_tlb_start,
+ * - drivers/xen/swiotlb-xen.c|418| <<xen_swiotlb_init>> free_bootmem(__pa(xen_io_tlb_start), PAGE_ALIGN(bytes));
+ * - drivers/xen/swiotlb-xen.c|420| <<xen_swiotlb_init>> free_pages((unsigned long )xen_io_tlb_start, order);
+ * - drivers/xen/swiotlb-xen.c|421| <<xen_swiotlb_init>> xen_io_tlb_start = NULL;
+ * - drivers/xen/swiotlb-xen.c|427| <<xen_swiotlb_init>> start_dma_addr = xen_virt_to_bus(xen_io_tlb_start);
+ * - drivers/xen/swiotlb-xen.c|429| <<xen_swiotlb_init>> if (swiotlb_init_with_tbl(xen_io_tlb_start, xen_io_tlb_nslabs,
+ * - drivers/xen/swiotlb-xen.c|434| <<xen_swiotlb_init>> rc = swiotlb_late_init_with_tbl(xen_io_tlb_start, xen_io_tlb_nslabs);
+ * - drivers/xen/swiotlb-xen.c|458| <<xen_swiotlb_init>> free_pages((unsigned long )xen_io_tlb_start, order);
+ *
+ * xen_io_tlb_end在以下使用:
+ * - drivers/xen/swiotlb-xen.c|194| <<is_xen_swiotlb_buffer>> paddr < virt_to_phys(xen_io_tlb_end);
+ * - drivers/xen/swiotlb-xen.c|407| <<xen_swiotlb_init>> xen_io_tlb_end = xen_io_tlb_start + bytes;
+ * - drivers/xen/swiotlb-xen.c|1015| <<xen_swiotlb_dma_supported>> return xen_virt_to_bus(xen_io_tlb_end - 1) <= mask;
+ *
+ * xen_io_tlb_start: swiotlb预留map用的起始虚拟地址
+ * xen_io_tlb_end: swiotlb预留map用的终止虚拟地址
+ *
+ * 在测试机器上 (start和end之间是64MB):
+ * xen_io_tlb_start = 0xffff8804a2a00000
+ * xen_io_tlb_end = 0xffff8804a6a00000
+ * xen_io_tlb_nslabs = 32768
+ */
static char *xen_io_tlb_start, *xen_io_tlb_end;
+/*
+ * used by:
+ * - drivers/xen/swiotlb-xen.c|290| <<xen_set_nslabs>> xen_io_tlb_nslabs = (64 * 1024 * 1024 >> IO_TLB_SHIFT);
+ * - drivers/xen/swiotlb-xen.c|291| <<xen_set_nslabs>> xen_io_tlb_nslabs = ALIGN(xen_io_tlb_nslabs, IO_TLB_SEGSIZE);
+ * - drivers/xen/swiotlb-xen.c|293| <<xen_set_nslabs>> xen_io_tlb_nslabs = nr_tbl;
+ * - drivers/xen/swiotlb-xen.c|295| <<xen_set_nslabs>> return xen_io_tlb_nslabs << IO_TLB_SHIFT;
+ * - drivers/xen/swiotlb-xen.c|357| <<xen_swiotlb_init>> xen_io_tlb_nslabs = swiotlb_nr_tbl();
+ * - drivers/xen/swiotlb-xen.c|364| <<xen_swiotlb_init>> bytes = xen_set_nslabs(xen_io_tlb_nslabs);
+ * - drivers/xen/swiotlb-xen.c|370| <<xen_swiotlb_init>> order = get_order(xen_io_tlb_nslabs << IO_TLB_SHIFT);
+ * - drivers/xen/swiotlb-xen.c|391| <<xen_swiotlb_init>> xen_io_tlb_nslabs = SLABS_PER_PAGE << order;
+ * - drivers/xen/swiotlb-xen.c|392| <<xen_swiotlb_init>> bytes = xen_io_tlb_nslabs << IO_TLB_SHIFT;
+ * - drivers/xen/swiotlb-xen.c|414| <<xen_swiotlb_init>> xen_io_tlb_nslabs);
+ * - drivers/xen/swiotlb-xen.c|429| <<xen_swiotlb_init>> if (swiotlb_init_with_tbl(xen_io_tlb_start, xen_io_tlb_nslabs,
+ * - drivers/xen/swiotlb-xen.c|434| <<xen_swiotlb_init>> rc = swiotlb_late_init_with_tbl(xen_io_tlb_start, xen_io_tlb_nslabs);
+ * - drivers/xen/swiotlb-xen.c|448| <<xen_swiotlb_init>> xen_io_tlb_nslabs = max(1024UL,
+ * - drivers/xen/swiotlb-xen.c|449| <<xen_swiotlb_init>> (xen_io_tlb_nslabs >> 1));
+ * - drivers/xen/swiotlb-xen.c|451| <<xen_swiotlb_init>> (xen_io_tlb_nslabs << IO_TLB_SHIFT) >> 20);
+ */
static unsigned long xen_io_tlb_nslabs;
/*
* Quick lookup value of the bus address of the IOTLB.
*/
+/*
+ * 在测试机上是0x1c0000
+ *
+ * 在xen_swiotlb_init()被配置成xen_virt_to_bus(xen_io_tlb_start)
+ */
static u64 start_dma_addr;
/*
@@ -108,6 +175,11 @@ static inline dma_addr_t xen_virt_to_bus(void *address)
return xen_phys_to_bus(virt_to_phys(address));
}
+/*
+ * 检查xen_pfn们对应的mfn是否机器连续
+ *
+ * 返回 1 说明连续
+ */
static int check_pages_physically_contiguous(unsigned long xen_pfn,
unsigned int offset,
size_t length)
@@ -126,6 +198,9 @@ static int check_pages_physically_contiguous(unsigned long xen_pfn,
return 1;
}
+/*
+ * 返回 0 说明连续
+ */
static inline int range_straddles_page_boundary(phys_addr_t p, size_t size)
{
unsigned long xen_pfn = XEN_PFN_DOWN(p);
@@ -133,11 +208,22 @@ static inline int range_straddles_page_boundary(phys_addr_t p, size_t size)
if (offset + size <= XEN_PAGE_SIZE)
return 0;
+ /*
+ * 检查xen_pfn们对应的mfn是否机器连续
+ * 返回1说明连续
+ */
if (check_pages_physically_contiguous(xen_pfn, offset, size))
return 0;
return 1;
}
+/*
+ * called by:
+ * - drivers/xen/swiotlb-xen.c|750| <<xen_unmap_single>> if (is_xen_swiotlb_buffer(dev_addr)) {
+ * - drivers/xen/swiotlb-xen.c|864| <<xen_swiotlb_sync_single>> if (is_xen_swiotlb_buffer(dev_addr))
+ *
+ * 判断地址是否在swiotlb预留范围内
+ */
static int is_xen_swiotlb_buffer(dma_addr_t dma_addr)
{
unsigned long bfn = XEN_PFN_DOWN(dma_addr);
@@ -155,23 +241,76 @@ static int is_xen_swiotlb_buffer(dma_addr_t dma_addr)
return 0;
}
+/*
+ * 只在以下使用:
+ * - drivers/xen/swiotlb-xen.c|241| <<xen_swiotlb_fixup>> } while (rc && dma_bits++ < max_dma_bits);
+ */
static int max_dma_bits = 32;
+/*
+ * called by only:
+ * - drivers/xen/swiotlb-xen.c|304| <<xen_swiotlb_init>> rc = xen_swiotlb_fixup(xen_io_tlb_start,
+ *
+ * 把预分配的页面换成连续的
+ *
+ * 在测试机上,
+ *
+ * buf = 0xffff8804a2a00000
+ * size = 64MB
+ * nslabs = 32768
+ */
static int
xen_swiotlb_fixup(void *buf, size_t size, unsigned long nslabs)
{
int i, rc;
int dma_bits;
dma_addr_t dma_handle;
+ /* 物理地址应该是0x4a2a00000 */
phys_addr_t p = virt_to_phys(buf);
+ /*
+ * IO_TLB_SEGSIZE: Maximum allowable number of contiguous slabs to map
+ * IO_TLB_SHIFT: log of the size of each IO TLB slab
+ *
+ * IO_TLB_SEGSIZE = 128, 左移11位是262144=0x40000 (256KB) ---> 1000000000000000000
+ * order是6, 加上PAGE_SHIFT后, dma_bits = 18
+ */
dma_bits = get_order(IO_TLB_SEGSIZE << IO_TLB_SHIFT) + PAGE_SHIFT;
+ /*
+ * 下面是2个循环
+ *
+ * 第一个循环是分配每一个连续的128 slot
+ *
+ * 假设默认是64MB = 65536KB, 每个slot是 2K, 一共65536 / 2 = 32768 slot
+ * 每次分配连续的128 slot, 所以一共分配 32768 / 128 = 256次
+ * 也就是说, 默认第一个循环跑256次
+ *
+ * 第二个循环的原因是每次分配可能会失败, 比如第一次在测试机器上分配128个slot的时候第四次才成功:
+ *
+ * [ 0.000000] xen:swiotlb_xen: orabug: xen_swiotlb_fixup() 2c400000, 6, 0x12
+ * [ 0.000000] xen:swiotlb_xen: orabug: xen_swiotlb_fixup() 2c400000, 6, 0x13
+ * [ 0.000000] xen:swiotlb_xen: orabug: xen_swiotlb_fixup() 2c400000, 6, 0x14
+ * [ 0.000000] xen:swiotlb_xen: orabug: xen_swiotlb_fixup() 2c400000, 6, 0x15
+ *
+ * 2c400000 : p + (i << IO_TLB_SHIFT)
+ * 6 : get_order(slabs << IO_TLB_SHIFT), --> 2^6 = 64 page = 128 slots
+ * 12 : dma_bits
+ *
+ * 一开始12个bit的mask分不到, 要0x15个才行
+ *
+ * 最后一次分配到了0x1b个bit的mask
+ * [ 0.000000] xen:swiotlb_xen: orabug: xen_swiotlb_fixup() 303c0000, 6, 0x1b
+ */
+
i = 0;
do {
int slabs = min(nslabs - i, (unsigned long)IO_TLB_SEGSIZE);
do {
+ /*
+ * 我们期待rc返回0
+ */
rc = xen_create_contiguous_region(
p + (i << IO_TLB_SHIFT),
get_order(slabs << IO_TLB_SHIFT),
@@ -184,6 +323,12 @@ xen_swiotlb_fixup(void *buf, size_t size, unsigned long nslabs)
} while (i < nslabs);
return 0;
}
+/*
+ * called by only:
+ * - drivers/xen/swiotlb-xen.c|322| <<xen_swiotlb_init>> bytes = xen_set_nslabs(xen_io_tlb_nslabs);
+ *
+ * nr_tlb在测试机是32768不为空, 左移11位返回67108864=0x4000000 (64MB)
+ */
static unsigned long xen_set_nslabs(unsigned long nr_tbl)
{
if (!nr_tbl) {
@@ -201,6 +346,11 @@ enum xen_swiotlb_err {
XEN_SWIOTLB_EFIXUP
};
+/*
+ * called by:
+ * - drivers/xen/swiotlb-xen.c|387| <<xen_swiotlb_init>> pr_err("%s (rc:%d)\n", xen_swiotlb_error(m_ret), rc);
+ * - drivers/xen/swiotlb-xen.c|389| <<xen_swiotlb_init>> panic("%s (rc:%d)", xen_swiotlb_error(m_ret), rc);
+ */
static const char *xen_swiotlb_error(enum xen_swiotlb_err err)
{
switch (err) {
@@ -216,6 +366,22 @@ static const char *xen_swiotlb_error(enum xen_swiotlb_err err)
}
return "";
}
+/*
+ * called by:
+ * - arch/x86/xen/pci-swiotlb-xen.c|60| <<pci_xen_swiotlb_init>> xen_swiotlb_init(1, true );
+ * - arch/x86/xen/pci-swiotlb-xen.c|77| <<pci_xen_swiotlb_init_late>> rc = xen_swiotlb_init(1, false );
+ *
+ * 在virtualbox上测试upstream linux, xen_swiotlb_init()只调用一次, early=true!
+ *
+ * [ 0.275532] xen_swiotlb_init+0x47/0x4e0
+ * [ 0.275538] pci_xen_swiotlb_init+0x18/0x29
+ * [ 0.275541] pci_iommu_alloc+0x5f/0x67
+ * [ 0.275543] mem_init+0xb/0x98
+ * [ 0.275546] start_kernel+0x23b/0x4da
+ * [ 0.275548] xen_start_kernel+0x561/0x56b
+ *
+ * verbose总是1
+ */
int __ref xen_swiotlb_init(int verbose, bool early)
{
unsigned long bytes, order;
@@ -223,13 +389,38 @@ int __ref xen_swiotlb_init(int verbose, bool early)
enum xen_swiotlb_err m_ret = XEN_SWIOTLB_UNKNOWN;
unsigned int repeat = 3;
+ /*
+ * 返回io_tlb_nslabs, 在测试机是32768
+ *
+ * xen_io_tlb_nslabs最终在测试机也是32768 (不知道后面逻辑有没有影响)
+ *
+ * 另外 在测试机上 (start和end之间是64MB):
+ * io_tlb_start: 0x4a2a00000
+ * io_tlb_end : 0x4a6a00000
+ */
+
xen_io_tlb_nslabs = swiotlb_nr_tbl();
retry:
+ /*
+ * nr_tlb在测试机是32768不为空, 左移11位返回67108864=0x4000000 (64MB)
+ *
+ * 因此bytes=64MB
+ *
+ * 函数只在这一处被调用!!!
+ */
bytes = xen_set_nslabs(xen_io_tlb_nslabs);
+ /*
+ * xen_io_tlb_nslabs左移11位是67108864=0x4000000 (64MB)
+ *
+ * order是14, 2^14是16384个4k的page, 总数是67108864
+ */
order = get_order(xen_io_tlb_nslabs << IO_TLB_SHIFT);
/*
* Get IO TLB memory from any location.
*/
+ /*
+ * 因为在upstream linux上early=true, 在bootmem分配64MB内存
+ */
if (early)
xen_io_tlb_start = alloc_bootmem_pages(PAGE_ALIGN(bytes));
else {
@@ -252,14 +443,24 @@ int __ref xen_swiotlb_init(int verbose, bool early)
m_ret = XEN_SWIOTLB_ENOMEM;
goto error;
}
+ /*
+ * 在测试机 上面bytes在xen_set_nslabs()推算的是64MB
+ *
+ * 于是, 我们在测试机得到如下结果 (start和end之间是64MB):
+ * xen_io_tlb_start = 0xffff8804a2a00000
+ * xen_io_tlb_end = 0xffff8804a6a00000
+ * xen_io_tlb_nslabs = 32768
+ */
xen_io_tlb_end = xen_io_tlb_start + bytes;
/*
* And replace that memory with pages under 4GB.
*/
+ /* 把预分配的页面换成连续的 */
rc = xen_swiotlb_fixup(xen_io_tlb_start,
bytes,
xen_io_tlb_nslabs);
if (rc) {
+ /* 下面是错误的时候 */
if (early)
free_bootmem(__pa(xen_io_tlb_start), PAGE_ALIGN(bytes));
else {
@@ -269,6 +470,7 @@ int __ref xen_swiotlb_init(int verbose, bool early)
m_ret = XEN_SWIOTLB_EFIXUP;
goto error;
}
+ /* start_dma_addr在测试机是0x1c0000 */
start_dma_addr = xen_virt_to_bus(xen_io_tlb_start);
if (early) {
if (swiotlb_init_with_tbl(xen_io_tlb_start, xen_io_tlb_nslabs,
@@ -281,6 +483,12 @@ int __ref xen_swiotlb_init(int verbose, bool early)
if (!rc)
swiotlb_set_max_segment(PAGE_SIZE);
+ /*
+ * 在测试机器上 (start和end之间是64MB):
+ * xen_io_tlb_start = 0xffff8804a2a00000
+ * xen_io_tlb_end = 0xffff8804a6a00000
+ * xen_io_tlb_nslabs = 32768
+ */
return rc;
error:
if (repeat--) {
@@ -298,6 +506,12 @@ int __ref xen_swiotlb_init(int verbose, bool early)
return rc;
}
+/*
+ * called by:
+ * - include/linux/dma-mapping.h|512| <<dma_alloc_attrs>> cpu_addr = ops->alloc(dev, size, dma_handle, flag, attrs);
+ *
+ * struct dma_map_ops xen_swiotlb_dma_ops.alloc = xen_swiotlb_alloc_coherent()
+ */
static void *
xen_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
dma_addr_t *dma_handle, gfp_t flags,
@@ -325,11 +539,24 @@ xen_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
* address. In fact on ARM virt_to_phys only works for kernel direct
* mapped RAM memory. Also see comment below.
*/
+ /* 就是调用__get_free_pages(), 然后转成物理地址存在dma_handle, 返回虚拟地址 */
ret = xen_alloc_coherent_pages(hwdev, size, dma_handle, flags, attrs);
+ /*
+ * 到了这里:
+ * ret 是虚拟地址
+ * dma_handle是物理地址
+ */
if (!ret)
return ret;
+ /*
+ * 如果dev->coherent_dma_mask已经有了就返回dev->coherent_dma_mask
+ * 否则根据gfp决定是DMA_BIT_MASK(24)还是DMA_BIT_MASK(32)
+ *
+ * 在这里, 如果dev->coherent_dma_mask已经有了就返回dev->coherent_dma_mask
+ * 否则用开始的32位
+ */
if (hwdev && hwdev->coherent_dma_mask)
dma_mask = dma_alloc_coherent_mask(hwdev, flags);
@@ -339,6 +566,10 @@ xen_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
* to *dma_handle. */
phys = *dma_handle;
dev_addr = xen_phys_to_bus(phys);
+ /*
+ * 如果地址在dma_mask范围内而且机器连续就直接使用
+ * 否则exchange成机器连续的
+ */
if (((dev_addr + size - 1 <= dma_mask)) &&
!range_straddles_page_boundary(phys, size))
*dma_handle = dev_addr;
@@ -353,6 +584,12 @@ xen_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
return ret;
}
+/*
+ * called by:
+ * - include/linux/dma-mapping.h|533| <<dma_free_attrs>> ops->free(dev, size, cpu_addr, dma_handle, attrs);
+ *
+ * struct dma_map_ops xen_swiotlb_dma_ops.free = xen_swiotlb_free_coherent()
+ */
static void
xen_swiotlb_free_coherent(struct device *hwdev, size_t size, void *vaddr,
dma_addr_t dev_addr, unsigned long attrs)
@@ -375,6 +612,7 @@ xen_swiotlb_free_coherent(struct device *hwdev, size_t size, void *vaddr,
range_straddles_page_boundary(phys, size))
xen_destroy_contiguous_region(phys, order);
+ /* 就是用free_pages()释放pages */
xen_free_coherent_pages(hwdev, size, vaddr, (dma_addr_t)phys, attrs);
}
@@ -385,12 +623,50 @@ xen_swiotlb_free_coherent(struct device *hwdev, size_t size, void *vaddr,
* Once the device is given the dma address, the device owns this memory until
* either xen_swiotlb_unmap_page or xen_swiotlb_dma_sync_single is performed.
*/
+/*
+ * 来自LDD
+ *
+ * Once a buffer has been mapped, it belongs to the device, not the processor. Until
+ * the buffer has been unmapped, the driver should not touch its contents in any
+ * way. Only after dma_unmap_single has been called is it safe for the driver to
+ * access the contents of the buffer.
+ *
+ * Occasionally a driver needs to access the contents of a streaming DMA buffer
+ * without unmapping it.
+ *
+ * dma_sync_single_for_cpu() should be called before the processor accesses a streaming
+ * DMA buffer. Once the call has been made, the CPU "owns" the DMA buffer and can work
+ * with it as needed.
+ *
+ * Before the device accesses the buffer, however, ownership should be transferred to
+ * the device with dma_sync_single_for_device(). The processor, once again, should not
+ * access the DMA buffer after this call has been made.
+ */
+/*
+ * e1000e一段代码例子:
+ * 693 buffer_info->dma = dma_map_single(&pdev->dev, skb->data,
+ * 694 adapter->rx_buffer_len,
+ * 695 DMA_FROM_DEVICE);
+ * 696 if (dma_mapping_error(&pdev->dev, buffer_info->dma)) {
+ * 697 dev_err(&pdev->dev, "Rx DMA map failed\n");
+ * 698 adapter->rx_dma_failed++;
+ * 699 break;
+ * 700 }
+ *
+ * called by:
+ * - include/linux/dma-mapping.h|233| <<dma_map_single_attrs>> addr = ops->map_page(dev, virt_to_page(ptr),
+ * - include/linux/dma-mapping.h|296| <<dma_map_page_attrs>> addr = ops->map_page(dev, page, offset, size, dir, attrs);
+ *
+ * struct dma_map_ops xen_swiotlb_dma_ops.map_page = xen_swiotlb_map_page()
+ */
static dma_addr_t xen_swiotlb_map_page(struct device *dev, struct page *page,
unsigned long offset, size_t size,
enum dma_data_direction dir,
unsigned long attrs)
{
+ /* phys是page中offset的物理地址 */
phys_addr_t map, phys = page_to_phys(page) + offset;
+ /* 物理地址转换成机器地址 */
dma_addr_t dev_addr = xen_phys_to_bus(phys);
BUG_ON(dir == DMA_NONE);
@@ -399,6 +675,13 @@ static dma_addr_t xen_swiotlb_map_page(struct device *dev, struct page *page,
* we can safely return the device addr and not worry about bounce
* buffering it.
*/
+ /*
+ * range_straddles_page_boundary()返回0说明连续
+ *
+ * xen_arch_need_swiotlb()对于x86永远是false
+ *
+ * 满足这些条件直接返回机器地址就可以了
+ */
if (dma_capable(dev, dev_addr, size) &&
!range_straddles_page_boundary(phys, size) &&
!xen_arch_need_swiotlb(dev, phys, dev_addr) &&
@@ -406,6 +689,7 @@ static dma_addr_t xen_swiotlb_map_page(struct device *dev, struct page *page,
/* we are not interested in the dma_addr returned by
* xen_dma_map_page, only in the potential cache flushes executed
* by the function. */
+ /* xen_dma_map_page()对于x86什么也不做 */
xen_dma_map_page(dev, page, dev_addr, offset, size, dir, attrs);
return dev_addr;
}
@@ -415,12 +699,19 @@ static dma_addr_t xen_swiotlb_map_page(struct device *dev, struct page *page,
*/
trace_swiotlb_bounced(dev, dev_addr, size, swiotlb_force);
+ /*
+ * map是物理地址
+ *
+ * 主要就是这个函数
+ */
map = swiotlb_tbl_map_single(dev, start_dma_addr, phys, size, dir,
attrs);
if (map == SWIOTLB_MAP_ERROR)
return XEN_SWIOTLB_ERROR_CODE;
+ /* map是物理地址 转换为机器地址 */
dev_addr = xen_phys_to_bus(map);
+ /* xen_dma_map_page()对于x86什么也不做 */
xen_dma_map_page(dev, pfn_to_page(map >> PAGE_SHIFT),
dev_addr, map & ~PAGE_MASK, size, dir, attrs);
@@ -444,6 +735,11 @@ static dma_addr_t xen_swiotlb_map_page(struct device *dev, struct page *page,
* After this call, reads by the cpu to the buffer are guaranteed to see
* whatever the device wrote there.
*/
+/*
+ * called by:
+ * - drivers/xen/swiotlb-xen.c|718| <<xen_swiotlb_unmap_page>> xen_unmap_single(hwdev, dev_addr, size, dir, attrs);
+ * - drivers/xen/swiotlb-xen.c|805| <<xen_swiotlb_unmap_sg_attrs>> xen_unmap_single(hwdev, sg->dma_address, sg_dma_len(sg), dir, attrs);
+ */
static void xen_unmap_single(struct device *hwdev, dma_addr_t dev_addr,
size_t size, enum dma_data_direction dir,
unsigned long attrs)
@@ -452,9 +748,11 @@ static void xen_unmap_single(struct device *hwdev, dma_addr_t dev_addr,
BUG_ON(dir == DMA_NONE);
+ /* xen_dma_unmap_page()对于x86什么也不做 */
xen_dma_unmap_page(hwdev, dev_addr, size, dir, attrs);
/* NOTE: We use dev_addr here, not paddr! */
+ /* 判断地址是否在swiotlb预留范围内 */
if (is_xen_swiotlb_buffer(dev_addr)) {
swiotlb_tbl_unmap_single(hwdev, paddr, size, dir, attrs);
return;
@@ -469,9 +767,17 @@ static void xen_unmap_single(struct device *hwdev, dma_addr_t dev_addr,
* are fine since dma_mark_clean() is null on POWERPC. We can
* make dma_mark_clean() take a physical address if necessary.
*/
+ /* 对于x86什么也不做 */
dma_mark_clean(phys_to_virt(paddr), size);
}
+/*
+ * called by:
+ * - include/linux/dma-mapping.h|251| <<dma_unmap_single_attrs>> ops->unmap_page(dev, addr, size, dir, attrs);
+ * - include/linux/dma-mapping.h|311| <<dma_unmap_page_attrs>> ops->unmap_page(dev, addr, size, dir, attrs);
+ *
+ * struct dma_map_ops xen_swiotlb_dma_ops.unmap_page = xen_swiotlb_unmap_page()
+ */
static void xen_swiotlb_unmap_page(struct device *hwdev, dma_addr_t dev_addr,
size_t size, enum dma_data_direction dir,
unsigned long attrs)
@@ -489,6 +795,56 @@ static void xen_swiotlb_unmap_page(struct device *hwdev, dma_addr_t dev_addr,
* address back to the card, you must first perform a
* xen_swiotlb_dma_sync_for_device, and then the device again owns the buffer
*/
+/*
+ * 来自LDD
+ *
+ * Once a buffer has been mapped, it belongs to the device, not the processor. Until
+ * the buffer has been unmapped, the driver should not touch its contents in any
+ * way. Only after dma_unmap_single has been called is it safe for the driver to
+ * access the contents of the buffer.
+ *
+ * Occasionally a driver needs to access the contents of a streaming DMA buffer
+ * without numapping it.
+ *
+ * dma_sync_single_for_cpu() should be called before the processor accesses a streaming
+ * DMA buffer. Once the call has been made, the CPU "owns" the DMA buffer and can work
+ * with it as needed.
+ *
+ * Before the device accesses the buffer, however, ownership should be transferred to
+ * the device with dma_sync_single_for_device(). The processor, once again, should not
+ * access the DMA buffer after this call has been made.
+ */
+/*
+ * sync的入口函数有4个
+ *
+ * .sync_single_for_cpu = xen_swiotlb_sync_single_for_cpu,
+ * .sync_single_for_device = xen_swiotlb_sync_single_for_device,
+ * .sync_sg_for_cpu = xen_swiotlb_sync_sg_for_cpu,
+ * .sync_sg_for_device = xen_swiotlb_sync_sg_for_device,
+ *
+ * xen_swiotlb_sync_single_for_cpu()
+ * -> xen_swiotlb_sync_single()