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: 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?

Even if it's not the case, it's suboptimal to have to write-lock and update the 
guest's P2M twice just to map a page of grants, which I will mention in the 
commit comment too.

  Paul

> 
> Jan


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