Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 9 Dec 2021 16:11:50 +0800 | From | Like Xu <> | Subject | Re: [PATCH v2 2/6] KVM: x86/pmu: Refactoring find_arch_event() to pmc_perf_hw_id() |
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On 9/12/2021 11:52 am, Jim Mattson wrote: > On Mon, Nov 29, 2021 at 11:42 PM Like Xu <like.xu.linux@gmail.com> wrote: >> >> From: Like Xu <likexu@tencent.com> >> >> The find_arch_event() returns a "unsigned int" value, >> which is used by the pmc_reprogram_counter() to >> program a PERF_TYPE_HARDWARE type perf_event. >> >> The returned value is actually the kernel defined generic > > Typo: generic. > >> perf_hw_id, let's rename it to pmc_perf_hw_id() with simpler >> incoming parameters for better self-explanation. >> >> Signed-off-by: Like Xu <likexu@tencent.com> >> --- >> arch/x86/kvm/pmu.c | 8 +------- >> arch/x86/kvm/pmu.h | 3 +-- >> arch/x86/kvm/svm/pmu.c | 8 ++++---- >> arch/x86/kvm/vmx/pmu_intel.c | 9 +++++---- >> 4 files changed, 11 insertions(+), 17 deletions(-) >> >> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c >> index 09873f6488f7..3b3ccf5b1106 100644 >> --- a/arch/x86/kvm/pmu.c >> +++ b/arch/x86/kvm/pmu.c >> @@ -174,7 +174,6 @@ static bool pmc_resume_counter(struct kvm_pmc *pmc) >> void reprogram_gp_counter(struct kvm_pmc *pmc, u64 eventsel) >> { >> unsigned config, type = PERF_TYPE_RAW; >> - u8 event_select, unit_mask; >> struct kvm *kvm = pmc->vcpu->kvm; >> struct kvm_pmu_event_filter *filter; >> int i; >> @@ -206,17 +205,12 @@ void reprogram_gp_counter(struct kvm_pmc *pmc, u64 eventsel) >> if (!allow_event) >> return; >> >> - event_select = eventsel & ARCH_PERFMON_EVENTSEL_EVENT; >> - unit_mask = (eventsel & ARCH_PERFMON_EVENTSEL_UMASK) >> 8; >> - >> if (!(eventsel & (ARCH_PERFMON_EVENTSEL_EDGE | >> ARCH_PERFMON_EVENTSEL_INV | >> ARCH_PERFMON_EVENTSEL_CMASK | >> HSW_IN_TX | >> HSW_IN_TX_CHECKPOINTED))) { > > The mechanics of the change look fine, but I do have some questions, > for my own understanding. > > Why don't we just use PERF_TYPE_RAW for guest counters all of the > time? What is the advantage of matching entries in a table so that we > can use PERF_TYPE_HARDWARE?
The first reason is we need PERF_TYPE_HARDWARE for fixed counters.
And then we might wonder whether we can create perf-event faster using PERF_TYPE_HARDWARE compared to PERF_TYPE_HARDWARE.
But the (current) answer is no, and probably the opposite:
# The cost (nanosecond) of calling perf_event_create_kernel_counter() PERF_TYPE_RAW Max= 1072211 Min= 11122 Avg= 41681.7
PERF_TYPE_HARDWARE Max= 46184215 Min= 16194 Avg= 250650
So why don't we just use PERF_TYPE_RAW for just all gp counters ?
Hi Peter, do you have any comments to invalidate this proposal ?
> > Why do the HSW_IN_TX* bits result in bypassing this clause, when these > bits are extracted as arguments to pmc_reprogram_counter below?
Once upon the time, the "PERF_TYPE_RAW" was introduced in the perf with comment "available TYPE space, raw is the max value", which means, per my understanding, it's our final type choice for creating a valid perf_event when HSW_IN_TX* bits are set and KVM needs to hack other perf_event_attr stuff for this HSW_IN_TX feature with the help of extracted arguments.
> > Reviewed-by: Jim Mattson <jmattson@google.com> >
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