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SubjectRe: [PATCH 1/2] KVM: x86: Update vPMCs when retiring instructions
On Tue, Nov 16, 2021 at 7:22 PM Like Xu <like.xu.linux@gmail.com> wrote:
>
> On 17/11/2021 6:15 am, Jim Mattson wrote:
> > On Tue, Nov 16, 2021 at 4:44 AM Like Xu <like.xu.linux@gmail.com> wrote:
> >>
> >> Hi Jim,
> >>
> >> On 13/11/2021 7:52 am, Jim Mattson wrote:
> >>> When KVM retires a guest instruction through emulation, increment any
> >>> vPMCs that are configured to monitor "instructions retired," and
> >>> update the sample period of those counters so that they will overflow
> >>> at the right time.
> >>>
> >>> Signed-off-by: Eric Hankland <ehankland@google.com>
> >>> [jmattson:
> >>> - Split the code to increment "branch instructions retired" into a
> >>> separate commit.
> >>> - Added 'static' to kvm_pmu_incr_counter() definition.
> >>> - Modified kvm_pmu_incr_counter() to check pmc->perf_event->state ==
> >>> PERF_EVENT_STATE_ACTIVE.
> >>> ]
> >>> Signed-off-by: Jim Mattson <jmattson@google.com>
> >>> Fixes: f5132b01386b ("KVM: Expose a version 2 architectural PMU to a guests")
> >>> ---
> >>> arch/x86/kvm/pmu.c | 31 +++++++++++++++++++++++++++++++
> >>> arch/x86/kvm/pmu.h | 1 +
> >>> arch/x86/kvm/x86.c | 3 +++
> >>> 3 files changed, 35 insertions(+)
> >>>
> >>> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> >>> index 09873f6488f7..153c488032a5 100644
> >>> --- a/arch/x86/kvm/pmu.c
> >>> +++ b/arch/x86/kvm/pmu.c
> >>> @@ -490,6 +490,37 @@ void kvm_pmu_destroy(struct kvm_vcpu *vcpu)
> >>> kvm_pmu_reset(vcpu);
> >>> }
> >>>
> >>> +static void kvm_pmu_incr_counter(struct kvm_pmc *pmc, u64 evt)
> >>> +{
> >>> + u64 counter_value, sample_period;
> >>> +
> >>> + if (pmc->perf_event &&
> >>
> >> We need to incr pmc->counter whether it has a perf_event or not.
> >>
> >>> + pmc->perf_event->attr.type == PERF_TYPE_HARDWARE &&
> >>
> >> We need to cover PERF_TYPE_RAW as well, for example,
> >> it has the basic bits for "{ 0xc0, 0x00, PERF_COUNT_HW_INSTRUCTIONS },"
> >> plus HSW_IN_TX or ARCH_PERFMON_EVENTSEL_EDGE stuff.
> >>
> >> We just need to focus on checking the select and umask bits:
> >
> > [What follows applies only to Intel CPUs. I haven't looked at AMD's
> > PMU implementation yet.]
>
> x86 has the same bit definition and semantics on at least the select and umask bits.

Yes, but AMD supports 12 bits of event selector. AMD also has the
HG_ONLY bits, which affect whether or not to count the event based on
context.

> >
> > Looking at the SDM, volume 3, Figure 18-1: Layout of IA32_PERFEVTSELx
> > MSRs, there seems to be a lot of complexity here, actually. In
>
> The devil is in the details.
>
> > addition to checking for the desired event select and unit mask, it
> > looks like we need to check the following:
> >
> > 1. The EN bit is set.
>
> We need to cover the EN bit of fixed counter 0 for HW_INSTRUCTIONS.

I don't know what you mean by that.

> > 2. The CMASK field is 0 (for events that can only happen once per cycle).
> > 3. The E bit is clear (maybe?).
>
> The "Edge detect" bit is about hw detail and let's ignore it.

From my reading of the SDM, I don't think the edge detect bit can be
ignored, but I will do some empirical tests to convince myself when I
get back from vacation.

> > 4. The OS bit is set if the guest is running at CPL0.
> > 5. The USR bit is set if the guest is running at CPL>0.
>
> CPL is a necessity.
>

As is host/guest mode on AMD.

> >
> >
> >> static inline bool eventsel_match_perf_hw_id(struct kvm_pmc *pmc,
> >> unsigned int perf_hw_id)
> >> {
> >> u64 old_eventsel = pmc->eventsel;
> >> unsigned int config;
> >>
> >> pmc->eventsel &=
> >> (ARCH_PERFMON_EVENTSEL_EVENT | ARCH_PERFMON_EVENTSEL_UMASK);
> >> config = kvm_x86_ops.pmu_ops->find_perf_hw_id(pmc);
> >> pmc->eventsel = old_eventsel;
> >> return config == perf_hw_id;
> >> }
>
> My proposal is to incr counter as long as the select and mask bits match the
> generi event.
>
> What do you think?
>
> >>
> >>> + pmc->perf_event->state == PERF_EVENT_STATE_ACTIVE &&
> >>
> >> Again, we should not care the pmc->perf_event.
> >
> > This test was intended as a proxy for checking that the counter is
> > enabled in the guest's IA32_PERF_GLOBAL_CTRL MSR.
>
> The two are not equivalent.

Yes. I'm getting that now.

> A enabled counter means true from "pmc_is_enabled(pmc) &&
> pmc_speculative_in_use(pmc)".
> A well-emulated counter means true from "perf_event->state ==
> PERF_EVENT_STATE_ACTIVE".
>
> A bad-emulated but enabled counter should be incremented for emulated instructions.

What is a "bad-emulated" counter?

> >
> >>> + pmc->perf_event->attr.config == evt) {
> >>
> >> So how about the emulated instructions for
> >> ARCH_PERFMON_EVENTSEL_USR and ARCH_PERFMON_EVENTSEL_USR ?
> >
> > I assume you're referring to the OS and USR bits of the corresponding
> > IA32_PERFEVTSELx MSR. I agree that these bits have to be consulted,
> > along with guest privilege level, before deciding whether or not to
> > count the event.
>
> Thanks and we may need update the testcase as well.

Indeed.

> >
> >>> + pmc->counter++;
> >>> + counter_value = pmc_read_counter(pmc);
> >>> + sample_period = get_sample_period(pmc, counter_value);
> >>> + if (!counter_value)
> >>> + perf_event_overflow(pmc->perf_event, NULL, NULL);
> >>
> >> We need to call kvm_perf_overflow() or kvm_perf_overflow_intr().
> >> And the patch set doesn't export the perf_event_overflow() SYMBOL.
> >
> > Oops. I was compiling with kvm built into vmlinux, so I missed this.
>
> In fact, I don't think the perf code would accept such rude symbolic export
> And I do propose to apply kvm_pmu_incr_counter() in a less invasive way.
>
> >
> >>> + if (local64_read(&pmc->perf_event->hw.period_left) >
> >>> + sample_period)
> >>> + perf_event_period(pmc->perf_event, sample_period);
> >>> + }
> >>> +}
> >>
> >> Not cc PeterZ or perf reviewers for this part of code is not a good thing.
> >
> > Added.
> >
> >> How about this:
> >>
> >> static void kvm_pmu_incr_counter(struct kvm_pmc *pmc)
> >> {
> >> struct kvm_pmu *pmu = pmc_to_pmu(pmc);
> >>
> >> pmc->counter++;
> >> reprogram_counter(pmu, pmc->idx);
> >> if (!pmc_read_counter(pmc))
> >> // https://lore.kernel.org/kvm/20211116122030.4698-1-likexu@tencent.com/T/#t
> >> kvm_pmu_counter_overflow(pmc, need_overflow_intr(pmc));
> >> }
> >>
> >>> +
> >>> +void kvm_pmu_record_event(struct kvm_vcpu *vcpu, u64 evt)
> >>
> >> s/kvm_pmu_record_event/kvm_pmu_trigger_event/
> >>
> >>> +{
> >>> + struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> >>> + int i;
> >>> +
> >>> + for (i = 0; i < pmu->nr_arch_gp_counters; i++)
> >>> + kvm_pmu_incr_counter(&pmu->gp_counters[i], evt);
> >>
> >> Why do we need to accumulate a counter that is not enabled at all ?
> >
> > In the original code, the condition checked in kmu_pmu_incr_counter()
> > was intended to filter out disabled counters.
>
> The bar of code review haven't been lowered, eh?

I have no idea what you mean. If anything, I'd like the bar for both
review and acceptance to be higher than it is today. No one was more
surprised than I was when Paolo accepted these patches so quickly.

> >
> >>> + for (i = 0; i < pmu->nr_arch_fixed_counters; i++)
> >>> + kvm_pmu_incr_counter(&pmu->fixed_counters[i], evt);
> >>
> >> How about this:
> >>
> >> for_each_set_bit(i, pmu->all_valid_pmc_idx, X86_PMC_IDX_MAX) {
> >> pmc = kvm_x86_ops.pmu_ops->pmc_idx_to_pmc(pmu, i);
> >>
> >> if (!pmc || !pmc_is_enabled(pmc) || !pmc_speculative_in_use(pmc))
> >> continue;
> >>
> >> // https://lore.kernel.org/kvm/20211116122030.4698-1-likexu@tencent.com/T/#t
> >> if (eventsel_match_perf_hw_id(pmc, perf_hw_id))
> >> kvm_pmu_incr_counter(pmc);
> >> }
> >>
> >
> > Let me expand the list of reviewers and come back with v2 after I
> > collect more input.
>
> I'm not sure Paolo will revert the "Queued both" decision,
> but I'm not taking my eyes or hands off the vPMU code.

I'm going on vacation for a couple of weeks. If Paolo doesn't want to
revert the buggy submissions from kvm-queue, then I will gladly defer
to you as the self-declared warden of the vPMU code to fix it as you
see fit.

Thanks!

--jim

> >
> > Thanks!
> >
> >
> >>> +}
> >>> +EXPORT_SYMBOL_GPL(kvm_pmu_record_event);
> >>> +
> >>> int kvm_vm_ioctl_set_pmu_event_filter(struct kvm *kvm, void __user *argp)
> >>> {
> >>> struct kvm_pmu_event_filter tmp, *filter;
> >>> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> >>> index 59d6b76203d5..d1dd2294f8fb 100644
> >>> --- a/arch/x86/kvm/pmu.h
> >>> +++ b/arch/x86/kvm/pmu.h
> >>> @@ -159,6 +159,7 @@ void kvm_pmu_init(struct kvm_vcpu *vcpu);
> >>> void kvm_pmu_cleanup(struct kvm_vcpu *vcpu);
> >>> void kvm_pmu_destroy(struct kvm_vcpu *vcpu);
> >>> int kvm_vm_ioctl_set_pmu_event_filter(struct kvm *kvm, void __user *argp);
> >>> +void kvm_pmu_record_event(struct kvm_vcpu *vcpu, u64 evt);
> >>>
> >>> bool is_vmware_backdoor_pmc(u32 pmc_idx);
> >>>
> >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> >>> index d7def720227d..bd49e2a204d5 100644
> >>> --- a/arch/x86/kvm/x86.c
> >>> +++ b/arch/x86/kvm/x86.c
> >>> @@ -7854,6 +7854,8 @@ int kvm_skip_emulated_instruction(struct kvm_vcpu *vcpu)
> >>> if (unlikely(!r))
> >>> return 0;
> >>>
> >>> + kvm_pmu_record_event(vcpu, PERF_COUNT_HW_INSTRUCTIONS);
> >>> +
> >>> /*
> >>> * rflags is the old, "raw" value of the flags. The new value has
> >>> * not been saved yet.
> >>> @@ -8101,6 +8103,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> >>> vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
> >>> if (!ctxt->have_exception ||
> >>> exception_type(ctxt->exception.vector) == EXCPT_TRAP) {
> >>> + kvm_pmu_record_event(vcpu, PERF_COUNT_HW_INSTRUCTIONS);
> >>> kvm_rip_write(vcpu, ctxt->eip);
> >>> if (r && (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
> >>> r = kvm_vcpu_do_singlestep(vcpu);
> >>>
> >

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