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

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Message-ID: <fbdcca61-e9c4-47fc-b629-7a46ad35cd24@intel.com>
Date: Mon, 1 Sep 2025 15:22:45 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Xin Li <xin@...or.com>, Binbin Wu <binbin.wu@...ux.intel.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 9/1/2025 3:04 PM, Xin Li wrote:
> 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.
> 

For handle_wrmsr_imm(), it seems we need to check 
guest_cpu_cap_has(X86_FEATURE_WRMSRNS) as well, since immediate form of 
MSR write is only supported on WRMSRNS instruction.

It leads to another topic, do we need to bother checking the opcode of 
the instruction on EXIT_REASON_MSR_WRITE and inject #UD when it is 
WRMSRNS instuction and !guest_cpu_cap_has(X86_FEATURE_WRMSRNS)?

WRMSRNS has virtualization hole as well, but KVM at least can emulate 
the architectural behavior when the write on MSRs are not pass through.