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[PATCH] x86: Use RDTSCP for synchronous get_cycles if possible

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RDTSCP is already synchronous and doesn't need an explicit CPUID.
This is a little faster and more importantly avoids VMEXITs on Hypervisors.

Original patch from Joerg Roedel, but reworked by AK
Also includes miscompilation fix by Eric Biederman

Cc: "Joerg Roedel" <joerg.roedel@amd.com>

Signed-off-by: Andi Kleen <ak@suse.de>
This commit is contained in:
Andi Kleen 2007-05-02 19:27:21 +02:00 committed by Andi Kleen
parent 9bccb23dc5
commit c5bcb5635a
1 changed files with 9 additions and 0 deletions
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9
include/asm-i386/tsc.h
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@ -37,6 +37,15 @@ static __always_inline cycles_t get_cycles_sync(void)
unsigned long long ret; unsigned long long ret;
unsigned eax; unsigned eax;
/*
* Use RDTSCP if possible; it is guaranteed to be synchronous
* and doesn't cause a VMEXIT on Hypervisors
*/
alternative_io(ASM_NOP3, ".byte 0x0f,0x01,0xf9", X86_FEATURE_RDTSCP,
"=A" (ret), "0" (0ULL) : "ecx", "memory");
if (ret)
return ret;
/* /*
* Don't do an additional sync on CPUs where we know * Don't do an additional sync on CPUs where we know
* RDTSC is already synchronous: * RDTSC is already synchronous: