Re: [patch 5/6] x86/fpu: Provide fpu_update_guest_xcr0/xfd()

[Paolo Bonzini]

On 12/16/21 06:36, Tian, Kevin wrote:
> 2) Do expansion at vCPU creation or KVM_ SET_CPUID2?
>
> If the reallocation concept is still kept, then we feel doing expansion in
> KVM_SET_CPUID2 makes slightly more sense. There is no functional
> difference between two options since the guest is not running at this
> point. And in general Qemu should set prctl according to the cpuid bits.
> But since anyway we still need to check guest cpuid against guest perm in
> KVM_SET_CPUID2, it reads clearer to expand the buffer only after this
> check is passed.

Yes, that makes sense to me as well.  In principle userspace could call 
prctl only after KVM_CREATE_VCPU.

> One option is to always disable WRMSR interception once 
> KVM_SET_CPUID2 succeeds, with the cost of one RDMSR per vm-exit. 
> But doing so affects legacy OS which even has no XFD logic at all.
>
> The other option is to continue the current policy i.e. disable write 
> emulation only after the 1st interception of setting XFD to a non-zero 
> value. Then the RDMSR cost is added only for guest which supports XFD.

For this I suggest to implement the current policy, but place it at the 
end of the series so it's easy to drop it.

Paolo