Re: [PATCH 00/10] x86/hvm: pkeys, add memory protection-key support

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 18/11/15 09:12, Wu, Feng wrote:
>
>> -----Original Message-----
>> From: xen-devel-bounces@lists.xen.org [mailto:xen-devel-
>> bounces@lists.xen.org] On Behalf Of Jan Beulich
>> Sent: Tuesday, November 17, 2015 6:26 PM
>> To: Andrew Cooper <andrew.cooper3@citrix.com>
>> Cc: Tian, Kevin <kevin.tian@intel.com>; wei.liu2@citrix.com;
>> ian.campbell@citrix.com; stefano.stabellini@eu.citrix.com;
>> george.dunlap@eu.citrix.com; ian.jackson@eu.citrix.com; xen-
>> devel@lists.xen.org; Nakajima, Jun <jun.nakajima@intel.com>; Han,
>> Huaitong <huaitong.han@intel.com>; keir@xen.org
>> Subject: Re: [Xen-devel] [PATCH 00/10] x86/hvm: pkeys, add memory
>> protection-key support
>>
>>>>> On 16.11.15 at 18:45, <andrew.cooper3@citrix.com> wrote:
>>> Furthermore, it is unclear (given the unwritten ABI) whether it is even
>>> safe to move _PAGE_GNTTAB out of the way, as this is visible to a PV guest.
>> It seems pretty clear to me that this would be unsafe: It being
>> part of L1_DISALLOW_MASK, if it moved and a guest used the
>> bit for its own purposes, the guest would break. I guess we'll
>> need an ELF note by which the guest can advertise which of the
>> available bits it doesn't care about itself.
> Actually, we don't expose this feature to PV guest, we only expose it
> to HVM. In that case, is there still issues like you discussed above?

You have turned on CR4.PKE, and _PAGE_GNTTAB is bit 62 in a PTE. 
Futhermore, you don't prevent/audit a PV guest's use of the PK bits.

This makes it usable by PV guests, even if the feature isn't advertised.

~Andrew