Re: [PATCH v2 2/6] KVM: x86/pmu: Refactoring find_arch_event() to pmc_perf_hw_id()

[Like Xu]

On 9/12/2021 11:52 am, Jim Mattson wrote:
> On Mon, Nov 29, 2021 at 11:42 PM Like Xu <like.xu.linux@xxxxxxxxx> wrote:
> > From: Like Xu <likexu@xxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxx>
> > ---
> >   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@xxxxxxxxxx>