Re: [RFC PATCH 03/13] kvm: Add XO memslot type

[Edgecombe, Rick P]

On Fri, 2019-10-04 at 09:27 +0200, Paolo Bonzini wrote:
> On 03/10/19 23:23, Rick Edgecombe wrote:
> > Add XO memslot type to create execute-only guest physical memory based on
> > the RO memslot. Like the RO memslot, disallow changing the memslot type
> > to/from XO.
> > 
> > In the EPT case ACC_USER_MASK represents the readable bit, so add the
> > ability for set_spte() to unset this.
> > 
> > This is based in part on a patch by Yu Zhang.
> > 
> > Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> 
> Instead of this, why not check the exit qualification gpa and, if it has
> the XO bit set, mask away both the XO bit and the R bit?  It can be done
> unconditionally for all memslots.  This should require no change to
> userspace.
> 
> Paolo
> 
The reasoning was that it seems like KVM leaves it to userspace to control the
physical address space layout since userspace decides the supported physical
address bits and lays out memory in the physical address space. So duplication
with XO memslots was an attempt was to keep the logic around that together.

I'll take another look at doing it this way though. I think userspace may still
need to adjust the MAXPHYADDR and be aware it can't layout memory in the XO
range.

Thanks,

Rick