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: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 26 September 2017 13:35
> To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Paul Durrant
> <Paul.Durrant@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: RE: [PATCH v7 02/12] x86/mm: add HYPERVISOR_memory_op to
> acquire guest resources
> 
> >>> On 26.09.17 at 14:20, <Paul.Durrant@xxxxxxxxxx> wrote:
> >>  -----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.
> 
> First of all - what would cause a mapping to appear at that slot (or in
> a range covering that slot). And then, while re-combining contiguous
> mappings indeed doesn't exist right now, replacing a non-leaf entry
> (page table) with a large page is very well supported (see e.g.
> ept_set_entry(), which even has a comment to that effect). Hence
> I continue to be confused why we need a new mechanism for
> seeding the grant tables.

I'll have to defer to Andrew to answer at this point.

  Paul

> 
> Jan


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