Re: [PATCH v6 3/6] KVM: selftests: Introduce __kvm_pmu_event_filter to improved event filter settings

[Sean Christopherson]

On Tue, Aug 15, 2023, Jinrong Liang wrote:
> Isaku Yamahata <isaku.yamahata@xxxxxxxxx> 于2023年8月15日周二 07:49写道：
> >
> > On Thu, Aug 10, 2023 at 05:09:42PM +0800,
> > Jinrong Liang <ljr.kernel@xxxxxxxxx> wrote:
> >
> > > From: Jinrong Liang <cloudliang@xxxxxxxxxxx>
> > >
> > > Add custom "__kvm_pmu_event_filter" structure to improve pmu event
> > > filter settings. Simplifies event filter setup by organizing event
> > > filter parameters in a cleaner, more organized way.
> > >
> > > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > Signed-off-by: Jinrong Liang <cloudliang@xxxxxxxxxxx>
> > > ---
> > >  .../kvm/x86_64/pmu_event_filter_test.c        | 182 +++++++++---------
> > >  1 file changed, 90 insertions(+), 92 deletions(-)
> > >
> > > diff --git a/tools/testing/selftests/kvm/x86_64/pmu_event_filter_test.c b/tools/testing/selftests/kvm/x86_64/pmu_event_filter_test.c
> > > index 5ac05e64bec9..94f5a89aac40 100644
> > > --- a/tools/testing/selftests/kvm/x86_64/pmu_event_filter_test.c
> > > +++ b/tools/testing/selftests/kvm/x86_64/pmu_event_filter_test.c
> > > @@ -28,6 +28,10 @@
> > >
> > >  #define NUM_BRANCHES 42
> > >
> > > +/* Matches KVM_PMU_EVENT_FILTER_MAX_EVENTS in pmu.c */
> > > +#define MAX_FILTER_EVENTS            300
> >
> > Can we simply use KVM_PMU_EVENT_FILTER_MAX_EVENTS and remove MAX_FILTER_EVENTS?
> 
> I didn't find the definition of KVM_PMU_EVENT_FILTER_MAX_EVENTS in
> selftests. KVM_PMU_EVENT_FILTER_MAX_EVENTS is defined in pmu.c. To use
> it, we need to define it in selftests.

Huh.  That seems like something that should be enumerated to userspace.

> > > +#define MAX_TEST_EVENTS              10
> > > +
> > >  /*
> > >   * This is how the event selector and unit mask are stored in an AMD
> > >   * core performance event-select register. Intel's format is similar,
> > > @@ -69,21 +73,33 @@
> > >
> > >  #define INST_RETIRED EVENT(0xc0, 0)
> > >
> > > +struct __kvm_pmu_event_filter {
> > > +     __u32 action;
> > > +     __u32 nevents;
> > > +     __u32 fixed_counter_bitmap;
> > > +     __u32 flags;
> > > +     __u32 pad[4];
> > > +     __u64 events[MAX_FILTER_EVENTS];
> > > +};
> > > +
> >
> > Is this same to struct kvm_pmu_event_filter?
> 
> In tools/arch/x86/include/uapi/asm/kvm.h
> 
> /* for KVM_CAP_PMU_EVENT_FILTER */
> struct kvm_pmu_event_filter {
> __u32 action;
> __u32 nevents;
> __u32 fixed_counter_bitmap;
> __u32 flags;
> __u32 pad[4];
> __u64 events[];
> };

To more directly answer Isaku's question:

They're *basically* the same, and have an identical layout, but the struct defined
by KVM uses a flexible array because the number of events comes from userspace
and forcing userspace to create an 1KiB+ object just to define a single event
filter would be obnoxious.

There are alternatives, e.g. using an struct overlay to set a single entry:

	struct {
		struct kvm_msrs header;
		struct kvm_msr_entry entry;
	} buffer = {};

	memset(&buffer, 0, sizeof(buffer));
	buffer.header.nmsrs = 1;
	buffer.entry.index = msr_index;
	buffer.entry.data = msr_value;

but that gets annoying (and IMO confusing) because of the nested structs.

I'll massage the changelog to callout the alternative, and why it's undesirable.