Re: [PATCH v3 1/1] x86/kvm/fpu: Mask guest fpstate->xfeatures with guest_supported_xcr0

[Paolo Bonzini]

On 2/16/22 08:48, Leonardo Bras Soares Passos wrote:
> On Mon, Feb 14, 2022 at 6:56 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> > On top of this patch, we can even replace vcpu->arch.guest_supported_xcr0
> > with vcpu->arch.guest_fpu.fpstate->user_xfeatures. Probably with local
> > variables or wrapper functions though, so as to keep the code readable.
>
> You mean another patch (#2) removing guest_supported_xcr0 field from
> kvm_vcpu_arch ?
> (and introducing something like kvm_guest_supported_xcr() ?)

Yes, introducing both kvm_guest_supported_xcr0() that just reads 
user_xfeatures, and kvm_guest_supported_xfd() as below.

> > For example:
> >
> > static inline u64 kvm_guest_supported_xfd()
> > {
> >          u64 guest_supported_xcr0 = vcpu->arch.guest_fpu.fpstate->user_xfeatures;
> >
> >          return guest_supported_xcr0 & XFEATURE_MASK_USER_DYNAMIC;
> > }
>
> Not sure If I get the above.
> Are you suggesting also removing fpstate->xfd and use a wrapper instead?
> Or is the above just an example?
> (s/xfd/xcr0/ & s/XFEATURE_MASK_USER_DYNAMIC/XFEATURE_MASK_USER_SUPPORTED/ )

The above is an example of how even "indirect" uses as 
guest_supported_xcr0 can be changed to a function.

> > Also, already in this patch fpstate_realloc should do
> >
> >           newfps->user_xfeatures = curfps->user_xfeatures | xfeatures;
> >
> > only if !guest_fpu.  In other words, the user_xfeatures of the guest FPU
> > should be controlled exclusively by KVM_SET_CPUID2.
>
> Just to check, you suggest adding this on patch #2 ?
> (I am failing to see how would that impact on #1)

In patch 1.  Since KVM_SET_CPUID2 now changes newfps->user_xfeatures, it 
should be the only place where it's changed, and arch_prctl() should not 
change it anymore.

Paolo