Re: [PATCH] x86/fpu: Remove dynamic features from xcomp_bv for init_fpstate

[Chang S. Bae]

On 10/13/2022 10:21 AM, Dave Hansen wrote:
> On 10/13/22 09:23, Chang S. Bae wrote:
> > --- a/arch/x86/kernel/fpu/xstate.c
> > +++ b/arch/x86/kernel/fpu/xstate.c
> > @@ -1127,8 +1127,12 @@ void __copy_xstate_to_uabi_buf(struct membuf to,
> > struct fpstate *fpstate,
> >           * non-compacted format disabled features still occupy state space,
> >           * but there is no state to copy from in the compacted
> >           * init_fpstate. The gap tracking will zero these states.
> > +        *
> > +        * In the case of guest fpstate, this user_xfeatures does not
> > +        * dynamically reflect the capacity of the XSAVE buffer but
> > +        * xfeatures does. So AND them together.
> >           */
> > -       mask = fpstate->user_xfeatures;
> > +       mask = fpstate->user_xfeatures & fpstate->xfeatures;
>
> I'm not sure this is quite right either.
>
> Doesn't kvm expect that all of the ->user_xfeatures will end up being
> copied out?  We surely can't copy them from 'fpstate' if the feature
> isn't there, but we can't skip them entirely, can we?

No, we can't skip them. IIUC, the code will zero out:

	/*
	 * ... The gap tracking will zero these states.
	 */
	mask = fpstate->user_xfeatures;

	for_each_extended_xfeature(i, mask) {
		/*
		 * If there was a feature or alignment gap, zero the space
		 * in the destination buffer.
		 */
		if (zerofrom < xstate_offsets[i])
			membuf_zero(&to, xstate_offsets[i] - zerofrom);

		<snip>

		/*
		 * Keep track of the last copied state in the non-compacted
		 * target buffer for gap zeroing.
		 */
		zerofrom = xstate_offsets[i] + xstate_sizes[i];
	}

out:
	if (to.left)
		membuf_zero(&to, to.left);

Thanks,
Chang