Re: [PATCH v10 10/28] x86/fpu/xstate: Update the XSTATE save function to support dynamic states

[Paolo Bonzini]

On 03/10/21 00:54, Bae, Chang Seok wrote:
> FWIW, the proposed KVM patch for AMX looks to take (1) here [1] and
> Paolo said [2]:
>
>      Most guests will not need the whole xstate feature set.  So perhaps you
>      could set XFD to the host value | the guest value, trap #NM if the
>      host XFD is zero, and possibly reflect the exception to the guest's XFD
                   ^^^^

(better: if the host XFD is nonzero, and the guest XCR0 includes any bit 
whose state is optional).

>      and XFD_ERR.

This comment is about letting arch/x86/kernel resize current->thread.fpu 
while the guest runs.  It's not necessary before KVM supports AMX, 
because KVM will never let a guest set XCR0[18] (__kvm_set_xcr).

Thomas instead was talking about allocation of vcpu->arch.guest_fpu and 
vcpu->arch.user_fpu.

For dynamic allocation in kvm_save_current_fpu, you can retrieve the 
XINUSE bitmask (XGETBV with RCX=1).  If it contains any bits that have 
optional state, you check if KVM's vcpu->arch.guest_fpu or 
vcpu->arch.user_fpu are already big enough, and if not do the reallocation.

If X86_FEATURE_XGETBV1 is not available, you will not need to resize. 
If XFD is supported but X86_FEATURE_XGETBV1 is not, you can just make 
kvm_arch_init fail with -ENODEV.  It's a nonsensical combination.

Thanks,

Paolo

>      In addition, loading the guest XFD MSRs should use the MSR autoload
>      feature (add_atomic_switch_msr).
>
> And then I guess discussion goes on..