Re: [PATCH 1/2] KVM: x86: Update vPMCs when retiring instructions

[Like Xu <like.xu.linux@gmail.com>]

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.

> 
> 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.

> 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.

> 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.

> 
> 
>> 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.

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.

> 
>>> +         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.

> 
>>> +             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?

> 
>>> +     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.

> 
> 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);
>>>
>