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Date: Mon, 22 Aug 2022 21:21:32 +0200 From: Andi Kleen <ak@...ux.intel.com> To: Sean Christopherson <seanjc@...gle.com>, Peter Zijlstra <peterz@...radead.org> Cc: "Liang, Kan" <kan.liang@...ux.intel.com>, acme@...hat.com, linux-kernel@...r.kernel.org, alexander.shishkin@...ux.intel.com, Jianfeng Gao <jianfeng.gao@...el.com>, Andrew Cooper <Andrew.Cooper3@...rix.com> Subject: Re: [RESEND PATCH] perf/x86/intel: Fix unchecked MSR access error for Alder Lake N > > Checking X86_FEATURE_HYPERVISOR in the guest won't handle things like trying to > run a non-hyrbid vCPU model on a hybrid CPU, because IIUC, the "is_hybrid()" is > purely based on FMS, i.e. will be false if someone enumerates a big core vCPU on > a hybrid CPU. > > So until KVM gets sane handling, shouldn't this be? > > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c > index f969410d0c90..0a8accfc3018 100644 > --- a/arch/x86/events/core.c > +++ b/arch/x86/events/core.c > @@ -2999,12 +2999,8 @@ void perf_get_x86_pmu_capability(struct x86_pmu_capability *cap) > } > > cap->version = x86_pmu.version; > - /* > - * KVM doesn't support the hybrid PMU yet. > - * Return the common value in global x86_pmu, > - * which available for all cores. > - */ > - cap->num_counters_gp = x86_pmu.num_counters; > + /* KVM doesn't support the hybrid PMU yet. */ > + cap->num_counters_gp = is_hybrid() ? 0 : x86_pmu.num_counters; That's just the PMU. Arguably if you don't handle hybrid affinity you shouldn't report the hybrid bit ever to the guest. So need more than that. But I guess Peter is concerned about the case when an old KVM is the host. I think a short term workaround for that is fine, but I don't think it's a good idea to completely disable it since that will break future setups with correct hybrid hypervisor too. We already probe the PMU MSRs, can't we detect this case there too? -Andi
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