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    SubjectRe: Performance overhead of get_cycles_sync
    From
    Ingo Molnar wrote:
    >
    > * Dor Laor <dor.laor@gmail.com> 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;
    }



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