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

To: Derek Murray <Derek.Murray@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: Next steps with pv_ops for Xen
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Wed, 05 Dec 2007 17:22:36 +0000
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Eduardo Habkost <ehabkost@xxxxxxxxxx>, Juan Quintela <quintela@xxxxxxxxxx>, "Stephen C. Tweedie" <sct@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxxxx>, Glauber de Oliveira Costa <gcosta@xxxxxxxxxx>, Chris Wright <chrisw@xxxxxxxxxxxx>, "virtualization@xxxxxxxxxxxxxx" <virtualization@xxxxxxxxxxxxxx>, Gerd Hoffmann <kraxel@xxxxxxxxxx>
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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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