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Date: Mon, 9 Mar 2020 08:44:45 -0700 From: Andi Kleen <ak@...ux.intel.com> To: Peter Zijlstra <peterz@...radead.org> Cc: "Liang, Kan" <kan.liang@...ux.intel.com>, Luwei Kang <luwei.kang@...el.com>, x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, mingo@...hat.com, acme@...nel.org, mark.rutland@....com, alexander.shishkin@...ux.intel.com, jolsa@...hat.com, namhyung@...nel.org, tglx@...utronix.de, bp@...en8.de, hpa@...or.com, pbonzini@...hat.com, sean.j.christopherson@...el.com, vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org, pawan.kumar.gupta@...ux.intel.com, thomas.lendacky@....com, fenghua.yu@...el.com, like.xu@...ux.intel.com Subject: Re: [PATCH v1 01/11] perf/x86/core: Support KVM to assign a dedicated counter for guest PEBS > Suppose your KVM thing claims counter 0/2 (ICL/SKL) for some random PEBS > event, and then the host wants to use PREC_DIST.. Then one of them will > be screwed for no reason what so ever. It's no different from some user using an event that requires some specific counter. > > How is that not destroying scheduling freedom? Any other situation we'd > have moved the !PREC_DIST PEBS event to another counter. Anyways what are you suggesting to do instead? Do you have a better proposal? The only alternative I know to doing this would be to go through the PEBS buffer in the guest and patch the applicable counter field up on each PMI. I tried that at some point (still have code somewhere), but it was quite complicated and tricky and somewhat slow, so I gave up eventually. It's also inherently racy because if the guest starts looking at the PEBS buffer before an PMI it could see the unpatched values Given I don't know any real software which would break from this, but such "polled PEBS" usages are certainly concievable. The artificial constraint is a lot simpler and straight forward, and also doesn't have any holes like this. -Andi
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