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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 5/12/07 17:17, "Derek Murray" <Derek.Murray@xxxxxxxxxxxx> wrote:

>> Actually I'm not so sure now. Presumably you add VM_PFNMAP to make
>> vm_normal_page() return NULL? But actually I would expect pte_pfn() to
>> return max_mapnr because the mapped page is not a local page. And that
>> should cause vm_normal_page() to return NULL always, regardless of whether
>> you assert VM_PFNMAP. Is gntdev being used to grant-and-map local pages in
>> the test that causes the crash?
> 
> That's right (gntdev is being used to map (but not grant) a local page).
> The test case creates a virtual block device in Dom0, and attempts to
> map its ring buffer in a user-space daemon in Dom0. Therefore pte_pfn
> succeeds.

Need to bite the bullet and fix this properly by setting a software flag in
ptes that are not subject to reference counting.

Unfortunately that also needs a hypervisor interface change, to allow
setting of those pte flags. Easily done though, and we should definitely get
that piece in for 3.2.0.

Setting VM_PFNMAP is bogus. We used to do that for privcmd mappings too, but
we stopped because IIRC it had other unwanted side effects.

 -- Keir



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