Re: [PATCH v7 05/26] x86/fpu/xstate: Introduce fpu_guest_cfg for guest FPU configuration

[Maxim Levitsky]

On Tue, 2023-11-28 at 14:58 +0000, Edgecombe, Rick P wrote:
> On Fri, 2023-11-24 at 00:53 -0500, Yang Weijiang wrote:
> > +       /*
> > +        * Set guest's __user_state_size to fpu_user_cfg.default_size
> > so that
> > +        * existing uAPIs can still work.
> > +        */
> > +       fpu->guest_perm.__user_state_size =
> > fpu_user_cfg.default_size;


> 
> It seems like an appropriate value, but where does this come into play
> exactly for guest FPUs?

It is used because permission API is used for guest fpu state as well (for user features),
and it affects two things:

1. If permission is not asked, then KVM will fail to resize the FPU state to match guest CPUID.
2. It will affect output size of the KVM_GET_XSAVE2 ioctl, which outputs buffer similar to
other FPU state buffers exposed to userspace (like one saved on signal stack, or one obtained via ptrace).


Best regards,
	Maxim Levitsky