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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,165 @@ | ||
| /* | ||
| * QEMU Thread Local Storage for coroutines | ||
| * | ||
| * Copyright Red Hat | ||
| * | ||
| * SPDX-License-Identifier: LGPL-2.1-or-later | ||
| * | ||
| * This work is licensed under the terms of the GNU LGPL, version 2.1 or later. | ||
| * See the COPYING.LIB file in the top-level directory. | ||
| * | ||
| * It is forbidden to access Thread Local Storage in coroutines because | ||
| * compiler optimizations may cause values to be cached across coroutine | ||
| * re-entry. Coroutines can run in more than one thread through the course of | ||
| * their life, leading bugs when stale TLS values from the wrong thread are | ||
| * used as a result of compiler optimization. | ||
| * | ||
| * An example is: | ||
| * | ||
| * ..code-block:: c | ||
| * :caption: A coroutine that may see the wrong TLS value | ||
| * | ||
| * static __thread AioContext *current_aio_context; | ||
| * ... | ||
| * static void coroutine_fn foo(void) | ||
| * { | ||
| * aio_notify(current_aio_context); | ||
| * qemu_coroutine_yield(); | ||
| * aio_notify(current_aio_context); // <-- may be stale after yielding! | ||
| * } | ||
| * | ||
| * This header provides macros for safely defining variables in Thread Local | ||
| * Storage: | ||
| * | ||
| * ..code-block:: c | ||
| * :caption: A coroutine that safely uses TLS | ||
| * | ||
| * QEMU_DEFINE_STATIC_CO_TLS(AioContext *, current_aio_context) | ||
| * ... | ||
| * static void coroutine_fn foo(void) | ||
| * { | ||
| * aio_notify(get_current_aio_context()); | ||
| * qemu_coroutine_yield(); | ||
| * aio_notify(get_current_aio_context()); // <-- safe | ||
| * } | ||
| */ | ||
|
|
||
| #ifndef QEMU_COROUTINE_TLS_H | ||
| #define QEMU_COROUTINE_TLS_H | ||
|
|
||
| /* | ||
| * To stop the compiler from caching TLS values we define accessor functions | ||
| * with __attribute__((noinline)) plus asm volatile("") to prevent | ||
| * optimizations that override noinline. | ||
| * | ||
| * The compiler can still analyze noinline code and make optimizations based on | ||
| * that knowledge, so an inline asm output operand is used to prevent | ||
| * optimizations that make assumptions about the address of the TLS variable. | ||
| * | ||
| * This is fragile and ultimately needs to be solved by a mechanism that is | ||
| * guaranteed to work by the compiler (e.g. stackless coroutines), but for now | ||
| * we use this approach to prevent issues. | ||
| */ | ||
|
|
||
| /** | ||
| * QEMU_DECLARE_CO_TLS: | ||
| * @type: the variable's C type | ||
| * @var: the variable name | ||
| * | ||
| * Declare an extern variable in Thread Local Storage from a header file: | ||
| * | ||
| * .. code-block:: c | ||
| * :caption: Declaring an extern variable in Thread Local Storage | ||
| * | ||
| * QEMU_DECLARE_CO_TLS(int, my_count) | ||
| * ... | ||
| * int c = get_my_count(); | ||
| * set_my_count(c + 1); | ||
| * *get_ptr_my_count() = 0; | ||
| * | ||
| * This is a coroutine-safe replacement for the __thread keyword and is | ||
| * equivalent to the following code: | ||
| * | ||
| * .. code-block:: c | ||
| * :caption: Declaring a TLS variable using __thread | ||
| * | ||
| * extern __thread int my_count; | ||
| * ... | ||
| * int c = my_count; | ||
| * my_count = c + 1; | ||
| * *(&my_count) = 0; | ||
| */ | ||
| #define QEMU_DECLARE_CO_TLS(type, var) \ | ||
| __attribute__((noinline)) type get_##var(void); \ | ||
| __attribute__((noinline)) void set_##var(type v); \ | ||
| __attribute__((noinline)) type *get_ptr_##var(void); | ||
|
|
||
| /** | ||
| * QEMU_DEFINE_CO_TLS: | ||
| * @type: the variable's C type | ||
| * @var: the variable name | ||
| * | ||
| * Define a variable in Thread Local Storage that was previously declared from | ||
| * a header file with QEMU_DECLARE_CO_TLS(): | ||
| * | ||
| * .. code-block:: c | ||
| * :caption: Defining a variable in Thread Local Storage | ||
| * | ||
| * QEMU_DEFINE_CO_TLS(int, my_count) | ||
| * | ||
| * This is a coroutine-safe replacement for the __thread keyword and is | ||
| * equivalent to the following code: | ||
| * | ||
| * .. code-block:: c | ||
| * :caption: Defining a TLS variable using __thread | ||
| * | ||
| * __thread int my_count; | ||
| */ | ||
| #define QEMU_DEFINE_CO_TLS(type, var) \ | ||
| static __thread type co_tls_##var; \ | ||
| type get_##var(void) { asm volatile(""); return co_tls_##var; } \ | ||
| void set_##var(type v) { asm volatile(""); co_tls_##var = v; } \ | ||
| type *get_ptr_##var(void) \ | ||
| { type *ptr = &co_tls_##var; asm volatile("" : "+rm" (ptr)); return ptr; } | ||
|
|
||
| /** | ||
| * QEMU_DEFINE_STATIC_CO_TLS: | ||
| * @type: the variable's C type | ||
| * @var: the variable name | ||
| * | ||
| * Define a static variable in Thread Local Storage: | ||
| * | ||
| * .. code-block:: c | ||
| * :caption: Defining a static variable in Thread Local Storage | ||
| * | ||
| * QEMU_DEFINE_STATIC_CO_TLS(int, my_count) | ||
| * ... | ||
| * int c = get_my_count(); | ||
| * set_my_count(c + 1); | ||
| * *get_ptr_my_count() = 0; | ||
| * | ||
| * This is a coroutine-safe replacement for the __thread keyword and is | ||
| * equivalent to the following code: | ||
| * | ||
| * .. code-block:: c | ||
| * :caption: Defining a static TLS variable using __thread | ||
| * | ||
| * static __thread int my_count; | ||
| * ... | ||
| * int c = my_count; | ||
| * my_count = c + 1; | ||
| * *(&my_count) = 0; | ||
| */ | ||
| #define QEMU_DEFINE_STATIC_CO_TLS(type, var) \ | ||
| static __thread type co_tls_##var; \ | ||
| static __attribute__((noinline, unused)) \ | ||
| type get_##var(void) \ | ||
| { asm volatile(""); return co_tls_##var; } \ | ||
| static __attribute__((noinline, unused)) \ | ||
| void set_##var(type v) \ | ||
| { asm volatile(""); co_tls_##var = v; } \ | ||
| static __attribute__((noinline, unused)) \ | ||
| type *get_ptr_##var(void) \ | ||
| { type *ptr = &co_tls_##var; asm volatile("" : "+rm" (ptr)); return ptr; } | ||
|
|
||
| #endif /* QEMU_COROUTINE_TLS_H */ |
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