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Date:   Tue, 26 May 2020 18:21:40 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Maxim Levitsky <mlevitsk@redhat.com>
Cc:     kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Tao Xu <tao3.xu@intel.com>,
        Jim Mattson <jmattson@google.com>,
        linux-kernel@vger.kernel.org, Joerg Roedel <joro@8bytes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Wanpeng Li <wanpengli@tencent.com>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Jingqi Liu <jingqi.liu@intel.com>
Subject: Re: [PATCH 2/2] kvm/x86: don't expose MSR_IA32_UMWAIT_CONTROL
 unconditionally
Message-ID: <20200527012140.GD31696@linux.intel.com>
References: <20200523161455.3940-1-mlevitsk@redhat.com>
 <20200523161455.3940-3-mlevitsk@redhat.com>
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On Sat, May 23, 2020 at 07:14:55PM +0300, Maxim Levitsky wrote:
> This msr is only available when the host supports WAITPKG feature.
> 
> This breaks a nested guest, if the L1 hypervisor is set to ignore
> unknown msrs, because the only other safety check that the
> kernel does is that it attempts to read the msr and
> rejects it if it gets an exception.
> 
> Fixes: 6e3ba4abce KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL

Same comments on the line wraps and Fixes tag.

For the code:

Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>

> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  arch/x86/kvm/x86.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b226fb8abe41b..4752293312947 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5316,6 +5316,10 @@ static void kvm_init_msr_list(void)
>  			    min(INTEL_PMC_MAX_GENERIC, x86_pmu.num_counters_gp))
>  				continue;
>  			break;
> +		case MSR_IA32_UMWAIT_CONTROL:
> +			if (!kvm_cpu_cap_has(X86_FEATURE_WAITPKG))
> +				continue;
> +			break;
>  		default:
>  			break;
>  		}
> -- 
> 2.26.2
>