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

[Xin Li <xin@zytor.com>]

On 9/2/2025 7:50 AM, Sean Christopherson wrote:
> On Mon, Sep 01, 2025, Xiaoyao Li wrote:
>> 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.
> 
> Hmm, no, inconsistent behavior (from the guest's perspective) is likely worse
> than eating with the virtualization hole.  Practically speaking, the only guest
> that's going to be surprised by the hole is a guest that's fuzzing opcodes, and
> a guest that's fuzzing opcodes at CPL0 isn't is going to create an inherently
> unstable environment no matter what.

Hmm, a malicious guest could *smartly* avoid causing such vmexits.  So more
or less that is wasteful and no objection here.

> 
> Though that raises the question of whether or not KVM should emulate WRMSRNS and
> whatever the official name for the "RDMSR with immediate" instruction is (I can't
> find it in the SDM).  I'm leaning "no", because outside of forced emulation, KVM
> should only "need" to emulate the instructions if Unrestricted Guest is disabled,
> the instructions should only be supported on CPUs with unrestricted guest, there's
> no sane reason (other than testing) to run a guest without Unrestricted Guest,
> and using the instructions in Big RM would be quite bizarre.  On the other hand,
> adding emulation support should be quite easy...
> 
> Side topic, does RDMSRLIST have any VMX controls?

Yes, bit 6 of Tertiary Processor-Based VM-Execution Controls, below is C&P
from Intel SDM:

Enable MSR-list instructions control: if this control is 0, any execution 
of RDMSRLIST or WRMSRLIST causes a #UD.

If the control is 1, the instruction commences normally, writing one MSR at
a time. Writes to certain MSRs are treated specially as described above for
WRMSR and WRMSRNS. In addition, attempts to access specific MSRs may cause 
VM exits.

For RDMSRLIST and WRMSRLIST, the exit qualification depends on the setting
of the “use MSR bitmaps” VM-execution control. If the control is 0, the
exit qualification is zero. If the control is 1, the exit qualification
is the index of the MSR whose access caused the VM exit.