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

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Message-ID: <92782a36-82ce-a76b-289b-3635f801d4a1@gmail.com>
Date:   Thu, 18 Nov 2021 19:26:32 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     "Paolo Bonzini - Distinguished Engineer (kernel-recipes.org)" 
        <pbonzini@...hat.com>, Eric Hankland <ehankland@...gle.com>,
        kvm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-perf-users@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        "Peter Zijlstra (Intel OTC, Netherlander)" <peterz@...radead.org>
Subject: Re: [PATCH 1/2] KVM: x86: Update vPMCs when retiring instructions

On 18/11/2021 11:37 am, Jim Mattson wrote:
> On Wed, Nov 17, 2021 at 12:01 PM Jim Mattson <jmattson@...gle.com> wrote:
>>
>> On Tue, Nov 16, 2021 at 7:22 PM Like Xu <like.xu.linux@...il.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@...il.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@...gle.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@...gle.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.
> 
> It looks like we already have an issue with event selector truncation
> on the AMD side. It's not clear from the APM if AMD has always had a
> 12-bit event selector field, but it's 12 bits now. Milan, for example,
> has at least 6 different events with selectors > 255. I don't see how
> a guest could monitor those events with the existing KVM
> implementation.

Yes and I have reproduced the issue on a Milan.
Thanks for your input, and let me try to fix it.

Thanks,
Like Xu