Re: [RFC 2/7] KVM: VMX: Expose IA32_PKRS MSR

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/02/21 10:53, Chenyi Qiang wrote:
>>>
>>
>> Is the guest expected to do a lot of reads/writes to the MSR (e.g. at 
>> every context switch)?
>>
>> Even if this is the case, the MSR intercepts and the entry/exit 
>> controls should only be done if CR4.PKS=1.  If the guest does not use 
>> PKS, KVM should behave as if these patches did not exist.
>>
> 
> Hi Paolo,
> 
> Per the MSR intercepts and entry/exit controls, IA32_PKRS access is 
> independent of the CR4.PKS bit, it just depends on CPUID enumeration. If 
> the guest doesn't set CR4.PKS but still has the CPUID capability, 
> modifying on PKRS should be supported but has no effect. IIUC, we can 
> not ignore these controls if CR4.PKS=0.

Understood, I wanted to avoid paying the price (if any) of loading PKRS 
on vmentry and vmexit not just if CPUID.PKS=0, but also if CR4.PKS=0. 
If CR4.PKS=0 it would be nicer to enable the MSR intercept and disable 
the vmentry/vmexit controls; just run the guest with the host value of 
IA32_PKRS.

Paolo