Re: [Xen-devel] Re: Next steps with pv_ops for Xen

[Jeremy Fitzhardinge <Jeremy.Fitzhardinge@xxxxxxxxxx>]

Derek Murray wrote:
> Keir Fraser wrote:
>> You'd need to track pte->grant_handle mappings somewhere, but it could
>> certainly be done this way, yes.
>
> At the moment, blktap and gntdev provide struct pages to
> get_user_pages by smuggling them in the vm_private_data field of the
> relevant vm_area_struct. Could we use this field to get the handles to
> ptep_get_and_clear_full as well?

Yes.  Given the mm and a vaddr passed to ptep_get_and_clear, find_vma()
will return the vma_struct.  If we assert that anyone who sets the "I'm
foreign" bit in a pte has a standard format for the vm_private_data
field, then we can stash a callback pointer there and make the
appropriate callback.

> Only downside that I can see is that we would need to find the vma for
> each PTE that needs to be cleared this way (since we don't get this
> passed to ptep_get_and_clear_full), but this is mitigated by (i) it
> only happening in the erroneous, unclean-shutdown case, and (ii)
> getting a hit in the mm->mmap_cache for consecutive runs of mapped
> grants.

Yes.  find_vma is fairly hot, since its used on every fault, so it
should be reasonably fast.  And it doesn't sound like our case is
particularly performance critical.

    J

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