linux-kernel - Re: [PATCH v3 4/6] KVM: x86: Add support for RDMSR/WRMSRNS w/ immediate on Intel

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Message-ID: <c4bc61da-c42c-453d-b484-f970b99cb616@zytor.com>
Date: Mon, 1 Sep 2025 00:04:24 -0700
From: Xin Li <xin@...or.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>, Xiaoyao Li <xiaoyao.li@...el.com>
Cc: Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v3 4/6] KVM: x86: Add support for RDMSR/WRMSRNS w/
 immediate on Intel

On 8/31/2025 11:34 PM, Binbin Wu wrote:
>> We need to inject #UD for !guest_cpu_has(X86_FEATURE_MSR_IMM)
>>
> 
> Indeed.

Good catch!

> 
> There is a virtualization hole of this feature for the accesses to the MSRs 
> not
> intercepted. IIUIC, there is no other control in VMX for this feature. If the
> feature is supported in hardware, the guest will succeed when it accesses 
> to the
> MSRs not intercepted even when the feature is not exposed to the guest, but 
> the
> guest will get #UD when access to the MSRs intercepted if KVM injects #UD.

hpa mentioned this when I just started the work.  But I managed to forget
it later... Sigh!

> 
> But I guess this is the guest's fault by not following the CPUID, KVM should
> still follow the spec?

I think we should still inject #UD when a MSR is intercepted by KVM.