Re: [Xen-devel] [PATCH v7 02/12] x86/mm: add HYPERVISOR_memory_op to acquire guest resources

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxx] On Behalf Of
> Paul Durrant
> Sent: 25 September 2017 16:00
> To: 'Jan Beulich' <JBeulich@xxxxxxxx>
> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] [PATCH v7 02/12] x86/mm: add
> HYPERVISOR_memory_op to acquire guest resources
> 
> > -----Original Message-----
> > From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> > Sent: 25 September 2017 15:23
> > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; xen-
> > devel@xxxxxxxxxxxxxxxxxxxx
> > Subject: Re: [PATCH v7 02/12] x86/mm: add HYPERVISOR_memory_op to
> > acquire guest resources
> >
> > >>> On 18.09.17 at 17:31, <paul.durrant@xxxxxxxxxx> wrote:
> > > Certain memory resources associated with a guest are not necessarily
> > > present in the guest P2M and so are not necessarily available to be
> > > foreign-mapped by a tools domain unless they are inserted, which risks
> > > shattering a super-page mapping.
> >
> > Btw., I'm additionally having trouble seeing this shattering of a
> > superpage: For one, xc_core_arch_get_scratch_gpfn() could be
> > a little less simplistic. And then even with the currently chosen
> > value (outside of the range of valid GFNs at that point in time)
> > there shouldn't be a larger page to be shattered, as there should
> > be no mapping at all at that index. But perhaps I'm just blind and
> > don't see the obvious ...
> 
> The shattering was Andrew's observation. Andrew, can you comment?
> 

Andrew commented verbally on this. It's not actually a shattering as such... 
The issue, apparently, is that adding the 4k grant table frame into the guest 
p2m will potentially cause creation of all layers of page table but removing it 
again will only remove the L1 entry. Thus it is no longer possible to use a 
superpage for that mapping at any point subsequently.

  Paul
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