linux-kernel - Re: [RESEND PATCH] KVM: x86/pmu: Support full width counting

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Message-ID: <aa182eec-3d0b-1452-19cc-20654190a2ae@redhat.com>
Date:   Mon, 4 May 2020 18:57:00 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Like Xu <like.xu@...ux.intel.com>,
        Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH] KVM: x86/pmu: Support full width counting

On 27/04/20 09:19, Like Xu wrote:
> +	if (vmx_supported_perf_capabilities())
> +		kvm_cpu_cap_check_and_set(X86_FEATURE_PDCM);

I think we can always set it, worst case it will be zero.

However, blocking intel_pmu_set_msr altogether is incorrect.  Instead,
you need to:

- list the MSR in msr_based_features_all so that it appears in
KVM_GET_MSR_FEATURE_INDEX_LIST

- return the supported bits in vmx_get_msr_feature

- allow host-initiated writes (as long as they only set supported bits)
of the MSR in intel_pmu_set_msr.

Thanks,

Paolo