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    SubjectRe: [PATCH] perf_event use rdpmc rather than rdmsr when possible in kernel
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    On Mon, 2012-02-20 at 17:38 -0500, Vince Weaver wrote:

    > diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
    > index 5adce10..5550047 100644
    > --- a/arch/x86/kernel/cpu/perf_event.c
    > +++ b/arch/x86/kernel/cpu/perf_event.c
    > @@ -85,7 +85,7 @@ u64 x86_perf_event_update(struct perf_event *event)
    > */
    > again:
    > prev_raw_count = local64_read(&hwc->prev_count);
    > - rdmsrl(hwc->event_base, new_raw_count);
    > + new_raw_count=native_read_pmc(hwc->event_base_rdpmc);

    That really wants to be rdpmc(), bypassing paravirt like that isn't
    nice.

    >
    > if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
    > new_raw_count) != prev_raw_count)
    > @@ -768,9 +768,11 @@ static inline void x86_assign_hw_event(struct perf_event *event,
    > } else if (hwc->idx >= X86_PMC_IDX_FIXED) {
    > hwc->config_base = MSR_ARCH_PERFMON_FIXED_CTR_CTRL;
    > hwc->event_base = MSR_ARCH_PERFMON_FIXED_CTR0 + (hwc->idx - X86_PMC_IDX_FIXED);
    > + hwc->event_base_rdpmc = (hwc->idx - X86_PMC_IDX_FIXED) | 1<<30;
    > } else {
    > hwc->config_base = x86_pmu_config_addr(hwc->idx);
    > hwc->event_base = x86_pmu_event_addr(hwc->idx);
    > + hwc->event_base_rdpmc = x86_pmu_addr_offset(hwc->idx);
    > }
    > }
    >
    > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
    > index abb2776..432ac69 100644
    > --- a/include/linux/perf_event.h
    > +++ b/include/linux/perf_event.h
    > @@ -562,6 +562,7 @@ struct hw_perf_event {
    > u64 last_tag;
    > unsigned long config_base;
    > unsigned long event_base;
    > + unsigned long event_base_rdpmc;

    We could make that unsigned int, the SDM explicitly states
    rdmsr/wrmsr/rdpmc take ECX (and ignore the upper 32bits RCX).

    > int idx;
    > int last_cpu;
    > struct hw_perf_event_extra extra_reg;

    But yeah, avoiding the extra variable will add a conditional and extra
    instructions to the rdpmc path.


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