Re: [PATCH 2/2] Documentation/x86: Explain guest XSTATE permission control

[Chang S. Bae]

On 6/16/2022 3:49 PM, Dave Hansen wrote:
> On 6/16/22 14:22, Chang S. Bae wrote:
> > +In addition, a couple of extended options are provided for a VCPU thread.
> > +The VCPU XSTATE permission is separately controlled.
> > +
> > +-ARCH_GET_XCOMP_GUEST_PERM
> > +
> > + arch_prctl(ARCH_GET_XCOMP_GUEST_PERM, &features);
> > +
> > + ARCH_GET_XCOMP_GUEST_PERM is a variant of ARCH_GET_XCOMP_PERM. So it
> > + provides the same semantics and functionality but for VCPU.
>
> This touches on the "what", but not the "why".  Could you explain in
> here both why this is needed and why an app might want to use it?

[ while studying on this a bit further, found a few things here ]

They (ARCH_{REQ|GET}_XCOMP_GUEST_PERM) provide a userspace VMM to 
request & check guest permission.

In general, KVM looks to have an API as a set of ioctls [1]. A guest VMM 
uses KVM_GET_DEVICE_ATTR::KVM_X86_XCOMP_GUEST_SUPP to query the 
available features [2][3]. ARCH_GET_XCOMP_SUPP is not usable here 
because KVM wants to control those exposed features [4] (via 
KVM_SUPPORTED_XCR0).

But oddly this mask does not appear to be actively referenced by those 
two arch_prctl options. I can see this ioctl attribute is currently 
disconnected from these arch_prctl options.

Also I failed to find the documentation about this 
KVM_X86_XCOMP_GUEST_SUPP interface:

	$ git grep KVM_X86_XCOMP_GUEST_SUPP ./Documentation/
	$

I guess people will be confused with having these two options only. I 
think documenting this has to come along with these missing pieces (and 
potential fix). So I'm inclined to drop this one at the moment.

Thanks,
Chang

[1] https://kernel.org/doc/html/latest/virt/kvm/index.html
[2] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/kvm/lib/x86_64/processor.c#n641
[3] 
https://github.com/qemu/qemu/blob/58b53669e87fed0d70903e05cd42079fbbdbc195/target/i386/kvm/kvm.c#L428
[4] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kvm/x86.c#n9008