Ingo Molnar wrote:
* Dor Laor <[email protected]> wrote:
> Hi Ingo, Thomas,
>
> In the latest kernel (2.6.24-rc3) I noticed a drastic performance
> decrease for KVM networking. The reason is many vmexit (exit reason is
> cpuid instruction) caused by calls to gettimeofday that uses tsc
> sourceclock. read_tsc calls get_cycles_sync which might call cpuid in
> order to serialize the cpu.
>
> Can you explain why the cpu needs to be serialized for every gettime
> call? Do we need to be that accurate? (It will also slightly improve
> physical hosts). I believe you have a reason and the answer is yes. In
> that case can you replace the serializing instruction with an
> instruction that does not trigger vmexit? Maybe use 'ltr' for example?
hm, where exactly does it call CPUID?
Ingo
Here, commented out [include/asm-x86/tsc.h]:
/* Like get_cycles, but make sure the CPU is synchronized. */
static __always_inline cycles_t get_cycles_sync(void)
{
unsigned long long ret;
unsigned eax, edx;
/*
* 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,
ASM_OUTPUT2("=a" (eax), "=d" (edx)),
"a" (0U), "d" (0U) : "ecx", "memory");
ret = (((unsigned long long)edx) << 32) | ((unsigned long long)eax);
if (ret)
return ret;
/*
* Don't do an additional sync on CPUs where we know
* RDTSC is already synchronous:
*/
// alternative_io("cpuid", ASM_NOP2, X86_FEATURE_SYNC_RDTSC,
// "=a" (eax), "0" (1) : "ebx","ecx","edx","memory");
rdtscll(ret);
return ret;
}
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
