RE: [PATCH v3 13/22] kvm: x86: Intercept #NM for saving IA32_XFD_ERR

[Tian, Kevin]

> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> Sent: Wednesday, December 29, 2021 8:10 AM
>
> But why even bother with an extra flag?  Can't
> vmx_update_exception_bitmap() get
> the guest's MSR_IA32_XFD value and intercept #NM accordingly?  Then you
> could
> even handle this fully in kvm_set_msr_common(), e.g.
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2c9606380bca..c6c936d2b298 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3704,6 +3704,8 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu,
> struct msr_data *msr_info)
>                         return 1;
> 
>                 fpu_update_guest_xfd(&vcpu->arch.guest_fpu, data);
> +               /* Blah blah blah blah */
> +               static_call(kvm_x86_update_exception_bitmap)(vcpu);
>                 break;

btw follow Paolo's suggestion here:

https://lore.kernel.org/all/6e95b6f7-44dc-7e48-4e6e-81cf85fc11c6@xxxxxxxxxx/

He prefers to disabling xfd interception in vmx specific code instead of
introducing a new callback to be called in common code. Since we want
to always trap #NM after xfd interception is disabled (which is done
after calling the msr common code), it also reads cleaner to call
update_exception_bitmap() from vmx.c instead of here.

Thanks
Kevin