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Date: Fri, 31 Dec 2021 12:00:37 +0800 From: Like Xu <like.xu.linux@...il.com> To: Sean Christopherson <seanjc@...gle.com> Cc: Paolo Bonzini <pbonzini@...hat.com>, Jim Mattson <jmattson@...gle.com>, Wanpeng Li <wanpengli@...cent.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, Joerg Roedel <joro@...tes.org>, Peter Zijlstra <peterz@...radead.org>, Like Xu <likexu@...cent.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH v11 01/17] perf/x86/intel: Add EPT-Friendly PEBS for Ice Lake Server On 31/12/2021 2:13 am, Sean Christopherson wrote: > On Fri, Dec 10, 2021, Like Xu wrote: >> From: Like Xu <like.xu@...ux.intel.com> >> >> From: Like Xu <like.xu@...ux.intel.com> > > Did one of these get handcoded? Uh, now I have found the use of "--from=<ident>". > >> The new hardware facility supporting guest PEBS is only available on >> Intel Ice Lake Server platforms for now. KVM will check this field >> through perf_get_x86_pmu_capability() instead of hard coding the cpu >> models in the KVM code. If it is supported, the guest PEBS capability >> will be exposed to the guest. > > So what exactly is this new feature? I've speed read the cover letter and a few > changelogs and didn't find anything that actually explained when this feature does. > Please check Intel SDM Vol3 18.9.5 for this "EPT-Friendly PEBS" feature. I assume when an unfamiliar feature appears in the patch SUBJECT, the reviewer may search for the exact name in the specification. > Based on the shortlog, I assume the feature handles translating linear addresses > via EPT? If that's correct, then x86_pmu.pebs_vmx should be named something like > x86_pmu.pebs_ept. "Translating linear addresses via EPT" is only part of the hardware implementation, and we may apply the new name if there are no other objections. > > That also raises the question of what will happen if EPT is disabled. Presumably > things will Just Work since no additional translation is needed, but if that's the > case then arguably vmx_pebs_supported() should be: > > return boot_cpu_has(X86_FEATURE_PEBS) && > (!tdp_enabled || kvm_pmu_cap.pebs_vmx); Yes, a similar fix is already on my private tree, and thank you for pointing it out! > > I'm guessing no one actually cares about supporting PEBS on older CPUs using shadow > paging, but the changelog should at least call out that PEBS is allowed if and only > if "pebs_vmx" is supported for simplicity, even though it would actually work if EPT > is disabled. And if for some reason it _doesn't_ work when EPT is disabled, then > vmx_pebs_supported() and friends need to actually check tdp_enabled. Yes, the guest PEBS only works when EPT is enabled on the newer modern CPUs. > > Regardless, this changelog really, really needs an explanation of the feature. Thank you for picking up, I will update the changelog for this commit. Please let me know if you have any more obstacles or niggles to review this patch set. Thanks, Like Xu
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