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

[Chang S. Bae]

On 6/23/2022 4:55 PM, Chang S. Bae wrote:
> On 6/16/2022 3:49 PM, Dave Hansen wrote:
> > 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.

Posted this series as following up this:

https://lore.kernel.org/lkml/20220823231402.7839-1-chang.seok.bae@xxxxxxxxx/

Thanks,
Chang