Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

[Thomas Gleixner]

On Thu, Oct 14 2021 at 16:09, Thomas Gleixner wrote:
> On Thu, Oct 14 2021 at 11:01, Paolo Bonzini wrote:
>
> Also you really should not wait until _all_ dynamic states are cleared
> in guest XFD. Because a guest which has bit 18 and 19 available but only
> uses one of them is going to trap on every other context switch due to
> XFD writes.
>
> So you check for
>
>    (guest_xfd & guest_perm) != guest_perm)
>
> and
>
>    (guest_xr0 & guest_perm) != 0
>
> If both are true, then you reallocate the buffers for _all_ permitted
> states _and_ set XFD to pass through.

And for that to work we must write XFD _before_ XSETBV in the guest boot
phase.

Thanks,

        tglx