Re: questions about rd{msr|tsc|pmc} instruction with x86-64

From: Jike Song
Date: Fri Oct 17 2008 - 01:38:49 EST

H. Peter Anvin wrote:
Jike Song wrote:
> Thanks, Peter! So I misunderstood the gcc constraint 'A' for x86-64,
> but seems the comment "while x86_64 returns at rax" still wrong,
> should this be fixed?

Yes, feel free to submit a patch.

Here you go... CC trivial@xxxxxxxxxx as well.

From 6eed2948d41f959dc113eb3ff30927bcacf34d08 Mon Sep 17 00:00:00 2001
From: Jike Song <albcamus@xxxxxxxxx>
Date: Fri, 17 Oct 2008 10:51:13 +0800
Subject: [PATCH] x86: correct wrong comment

The rdmsr instruction(et al) for i386 and x86-64 are semantically same.
The only difference is how gcc interpret constraint "A" for these targets.

Signed-off-by: Jike Song <albcamus@xxxxxxxxx>
include/asm-x86/msr.h | 8 ++++----
1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/asm-x86/msr.h b/include/asm-x86/msr.h
index 530af1f..fd0e1a1 100644
--- a/include/asm-x86/msr.h
+++ b/include/asm-x86/msr.h
@@ -22,10 +22,10 @@ static inline unsigned long long native_read_tscp(unsigned int *aux)

- * i386 calling convention returns 64-bit value in edx:eax, while
- * x86_64 returns at rax. Also, the "A" constraint does not really
- * mean rdx:rax in x86_64, so we need specialized behaviour for each
- * architecture
+ * 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.
#ifdef CONFIG_X86_64
#define DECLARE_ARGS(val, low, high) unsigned low, high

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