linux-kernel - Re: [PATCH v6 04/12] KVM: vmx/pmu: Emulate MSR_ARCH_LBR

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Date:   Fri, 16 Jul 2021 16:33:36 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
        wei.w.wang@...el.com, like.xu.linux@...il.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH v6 04/12] KVM: vmx/pmu: Emulate MSR_ARCH_LBR_DEPTH for
 guest Arch LBR

On Fri, Jul 16, 2021 at 1:36 AM Yang Weijiang <weijiang.yang@...el.com> wrote:
>
> From: Like Xu <like.xu@...ux.intel.com>
>
> The number of Arch LBR entries available is determined by the value
> in host MSR_ARCH_LBR_DEPTH.DEPTH. The supported LBR depth values are
> enumerated in CPUID.(EAX=01CH, ECX=0):EAX[7:0]. For each bit "n" set
> in this field, the MSR_ARCH_LBR_DEPTH.DEPTH value of "8*(n+1)" is
> supported.
>
> On a guest write to MSR_ARCH_LBR_DEPTH, all LBR entries are reset to 0.
> KVM writes guest requested value to the native ARCH_LBR_DEPTH MSR
> (this is safe because the two values will be the same) when the Arch LBR
> records MSRs are pass-through to the guest.
>
> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>

It might be worth noting that KVM_SET_MSRS cannot be used to emulate a
wrmsr instruction in the guest, but maybe that's already implicit.

Reviewed-by: Jim Mattson <jmattson@...gle.com>