RE: [PATCH v2 18/23] kvm: x86: Get/set expanded xstate buffer

[Wang, Wei W]

On Monday, December 20, 2021 5:04 PM, Paolo Bonzini wrote:
> On 12/17/21 16:29, Jing Liu wrote:
> > +/* for KVM_CAP_XSAVE and KVM_CAP_XSAVE2 */
> >   struct kvm_xsave {
> > +	/*
> > +	 * KVM_GET_XSAVE only uses the first 4096 bytes.
> > +	 *
> > +	 * KVM_GET_XSAVE2 must have the size match what is returned by
> > +	 * KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2).
> > +	 *
> > +	 * KVM_SET_XSAVE uses the extra field if guest_fpu::fpstate::size
> > +	 * exceeds 4096 bytes.
> 
> KVM_GET_XSAVE2 and KVM_SET_XSAVE respectively write and read as many
> bytes as are returned by KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2), when
> invoked on the vm file descriptor.  Currently,
> KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will only return a value that is
> greater than 4096 bytes if any dynamic features have been enabled with
> ``arch_prctl()``; this however may change in the future.

Would this make people think that KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) doesn’t
return the value (i.e. return 0) if it is smaller than 4096?
(i.e. KVM_GET_XSAVE2 doesn't work with size < 4096, which isn’t true)

I plan to just reword a bit:
Currently, KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will only return a size value,
and the value is greater than 4096 bytes if any dynamic features have been enabled with
``arch_prctl()``. More types of values could be returned in the future.

Thanks,
Wei