RE: [patch 4/6] x86/fpu: Add guest support to xfd_enable_feature()

[Tian, Kevin]

> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Sent: Wednesday, December 15, 2021 5:53 PM
> 
> On Wed, Dec 15 2021 at 05:46, Kevin Tian wrote:
> >> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >> +	if (guest_fpu) {
> >> +		newfps->is_guest = true;
> >> +		newfps->is_confidential = curfps->is_confidential;
> >> +		newfps->in_use = curfps->in_use;
> >
> > What is the purpose of this 'in_use' field? Currently it's only
> > touched in three places:
> >
> >   - set when entering guest;
> >   - cleared when exiting to userspace;
> >   - checked when freeing a guest FPU;
> >
> > The last one can be easily checked by comparing to current fps.
> 
> I added it for paranoia sake because the destruction of the KVM FPU
> state is not necessarily in the context of the vCPU thread. Yes, it
> should not happen...
> 
> >> +	if (guest_fpu) {
> >> +		curfps = xchg(&guest_fpu->fpstate, newfps);
> >
> > This can be a direct value update to guest_fpu->fpstate since
> > curfps has already been acquired in the start.
> 
> Indeed.
> 

Thanks for confirmation. We'll include those changes in next version.

Thanks
Kevin