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Subject: Re: [PATCH 1/2] KVM: X86: Move ignore_msrs handling upper the stack
To:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Peter Xu <peterx@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Vitaly Kuznetsov <vkuznets@redhat.com>
References: <20200622220442.21998-1-peterx@redhat.com>
 <20200622220442.21998-2-peterx@redhat.com>
 <20200625061544.GC2141@linux.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <1cebc562-89e9-3806-bb3c-771946fc64f3@redhat.com>
Date:   Thu, 25 Jun 2020 10:09:13 +0200
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On 25/06/20 08:15, Sean Christopherson wrote:
> IMO, kvm_cpuid() is simply buggy.  If KVM attempts to access a non-existent
> MSR then it darn well should warn.
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 8a294f9747aa..7ef7283011d6 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -1013,7 +1013,8 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>                 *ebx = entry->ebx;
>                 *ecx = entry->ecx;
>                 *edx = entry->edx;
> -               if (function == 7 && index == 0) {
> +               if (function == 7 && index == 0 && (*ebx | (F(RTM) | F(HLE))) &&
> +                   (vcpu->arch.arch_capabilities & ARCH_CAP_TSX_CTRL_MSR)) {
>                         u64 data;
>                         if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
>                             (data & TSX_CTRL_CPUID_CLEAR))
> 

That works too, but I disagree that warning is the correct behavior
here.  It certainly should warn as long as kvm_get_msr blindly returns
zero.  However, for a guest it's fine to access a potentially
non-existent MSR if you're ready to trap the #GP, and the point of this
series is to let cpuid.c or any other KVM code do the same.

Paolo