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x86/msr: Improve the comments of the DECLARE_ARGS()/EAX_EDX_VAL()/EAX_EDX_RET() facility

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Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: linux-kernel@vger.kernel.org
This commit is contained in:
Ingo Molnar
2025-05-02 10:07:59 +02:00
parent 0c7b20b852
commit 76deb5452e
1 changed files with 13 additions and 11 deletions
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24
arch/x86/include/asm/msr.h
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@@ -37,20 +37,22 @@ struct saved_msrs {
};
/*
* both i386 and x86_64 returns 64-bit value in edx:eax, but gcc's "A"
* constraint has different meanings. For i386, "A" means exactly
* edx:eax, while for x86_64 it doesn't mean rdx:rax or edx:eax. Instead,
* it means rax *or* rdx.
* Both i386 and x86_64 returns 64-bit values in edx:eax for certain
* instructions, but GCC's "A" constraint has different meanings.
* For i386, "A" means exactly edx:eax, while for x86_64 it
* means rax *or* rdx.
*
* These helpers wrapping these semantic differences save one instruction
* clearing the high half of 'low':
*/
#ifdef CONFIG_X86_64
/* Using 64-bit values saves one instruction clearing the high half of low */
#define DECLARE_ARGS(val, low, high) unsigned long low, high
#define EAX_EDX_VAL(val, low, high) ((low) | (high) << 32)
#define EAX_EDX_RET(val, low, high) "=a" (low), "=d" (high)
# define DECLARE_ARGS(val, low, high) unsigned long low, high
# define EAX_EDX_VAL(val, low, high) ((low) | (high) << 32)
# define EAX_EDX_RET(val, low, high) "=a" (low), "=d" (high)
#else
#define DECLARE_ARGS(val, low, high) u64 val
#define EAX_EDX_VAL(val, low, high) (val)
#define EAX_EDX_RET(val, low, high) "=A" (val)
# define DECLARE_ARGS(val, low, high) u64 val
# define EAX_EDX_VAL(val, low, high) (val)
# define EAX_EDX_RET(val, low, high) "=A" (val)
#endif
/*