linux-kernel - Re: [PATCH v6 07/16] KVM: x86/pmu: Reprogram PEBS event to emulate guest PEBS counter

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Message-ID: <ff5a419f-188f-d14c-72c8-4b760052734d@linux.intel.com>
Date:   Mon, 17 May 2021 07:44:15 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Like Xu <like.xu@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, weijiang.yang@...el.com,
        Kan Liang <kan.liang@...ux.intel.com>, wei.w.wang@...el.com,
        eranian@...gle.com, liuxiangdong5@...wei.com,
        linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v6 07/16] KVM: x86/pmu: Reprogram PEBS event to emulate
 guest PEBS counter


On 5/17/2021 1:39 AM, Peter Zijlstra wrote:
> On Tue, May 11, 2021 at 10:42:05AM +0800, Like Xu wrote:
>> +	if (pebs) {
>> +		/*
>> +		 * The non-zero precision level of guest event makes the ordinary
>> +		 * guest event becomes a guest PEBS event and triggers the host
>> +		 * PEBS PMI handler to determine whether the PEBS overflow PMI
>> +		 * comes from the host counters or the guest.
>> +		 *
>> +		 * For most PEBS hardware events, the difference in the software
>> +		 * precision levels of guest and host PEBS events will not affect
>> +		 * the accuracy of the PEBS profiling result, because the "event IP"
>> +		 * in the PEBS record is calibrated on the guest side.
>> +		 */
>> +		attr.precise_ip = 1;
>> +	}
> You've just destroyed precdist, no?

precdist can mean multiple things:

- Convert cycles to the precise INST_RETIRED event. That is not 
meaningful for virtualization because "cycles" doesn't exist, just the 
raw events.

- For GLC+ and TNT+ it will force the event to a specific counter that 
is more precise. This would be indeed "destroyed", but right now the 
patch kit only supports Icelake which doesn't support that anyways.

So I think the code is correct for now, but will need to be changed for 
later CPUs. Should perhaps fix the comment though to discuss this.


-Andi