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

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Wednesday, November 18, 2015 6:11 PM
> To: Wu, Feng <feng.wu@xxxxxxxxx>; Jan Beulich <JBeulich@xxxxxxxx>
> Cc: Tian, Kevin <kevin.tian@xxxxxxxxx>; wei.liu2@xxxxxxxxxx;
> ian.campbell@xxxxxxxxxx; stefano.stabellini@xxxxxxxxxxxxx;
> george.dunlap@xxxxxxxxxxxxx; ian.jackson@xxxxxxxxxxxxx; xen-
> devel@xxxxxxxxxxxxx; Nakajima, Jun <jun.nakajima@xxxxxxxxx>; Han,
> Huaitong <huaitong.han@xxxxxxxxx>; keir@xxxxxxx
> Subject: Re: [Xen-devel] [PATCH 00/10] x86/hvm: pkeys, add memory
> protection-key support
> 
> On 18/11/15 09:12, Wu, Feng wrote:
> >
> >> -----Original Message-----
> >> From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> >> bounces@xxxxxxxxxxxxx] On Behalf Of Jan Beulich
> >> Sent: Tuesday, November 17, 2015 6:26 PM
> >> To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> >> Cc: Tian, Kevin <kevin.tian@xxxxxxxxx>; wei.liu2@xxxxxxxxxx;
> >> ian.campbell@xxxxxxxxxx; stefano.stabellini@xxxxxxxxxxxxx;
> >> george.dunlap@xxxxxxxxxxxxx; ian.jackson@xxxxxxxxxxxxx; xen-
> >> devel@xxxxxxxxxxxxx; Nakajima, Jun <jun.nakajima@xxxxxxxxx>; Han,
> >> Huaitong <huaitong.han@xxxxxxxxx>; keir@xxxxxxx
> >> Subject: Re: [Xen-devel] [PATCH 00/10] x86/hvm: pkeys, add memory
> >> protection-key support
> >>
> >>>>> On 16.11.15 at 18:45, <andrew.cooper3@xxxxxxxxxx> 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.

Oh, yes, actually, we shouldn't turn on CR4.PKE for Xen, since we don't
actually enable it for Xen itself (No such usage model).

> Futhermore, you don't prevent/audit a PV guest's use of the PK bits.

I think the guest (HVM or PV) should use the PK bits only when Pkey
is enabled (CR4.PKE set) by the kernel, Xen cannot control it, right?

Thanks,
Feng

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

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