linux-kernel - Re: [PATCH] KVM: VMX: Always treat MSR_IA32_PERF

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Date:   Thu, 4 Jun 2020 18:49:44 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        "Xu, Like" <like.xu@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH] KVM: VMX: Always treat MSR_IA32_PERF_CAPABILITIES as a
 valid PMU MSR

On 04/06/20 18:44, Jim Mattson wrote:
>>> I don't know if I would call it a "good assumption" so much as a "necessary
>>> assumption".  KVM_{GET,SET}_MSRS are allowed, and must function correctly,
>>> if they're called prior to KVM_SET_CPUID{2}.
>> Generally speaking this is not the case for the PMU; get_gp_pmc for
>> example depends on pmu->nr_arch_gp_counters which is initialized based
>> on CPUID leaf 0xA.
>>
>> The assumption that this patch fixes is that you can blindly take the
>> output of KVM_GET_MSR_INDEX_LIST and pass it to KVM_{GET,SET}_MSRS.
> 
> Is that an assumption or an invariant?

Both, I guess (a valid assumption for userspace, an invariant to be
respected for the kernel code).

The part where we don't fare to well, is that a bunch of MSRs that need
save/restore are _not_ included in KVM_GET_MSR_INDEX_LIST (and the PMU
is the biggest if not the only offender there).

Paolo