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

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Date:   Tue, 23 Aug 2022 16:34:26 -0700
From:   "Chang S. Bae" <chang.seok.bae@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>, <len.brown@...el.com>,
        <tony.luck@...el.com>, <rafael.j.wysocki@...el.com>,
        <reinette.chatre@...el.com>, <dan.j.williams@...el.com>
CC:     <corbet@....net>, <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] Documentation/x86: Explain guest XSTATE permission
 control

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@intel.com/

Thanks,
Chang