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

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: Next steps with pv_ops for Xen
From: Derek Murray <Derek.Murray@xxxxxxxxxxxx>
Date: Wed, 05 Dec 2007 17:48:49 +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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Keir Fraser wrote:
Need to bite the bullet and fix this properly by setting a software flag in
ptes that are not subject to reference counting.

Could we get away with testing the VM_FOREIGN flag in vm_normal_page()? Although I get the impression that this wouldn't be easily justified if trying to merge with upstream Linux....

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.

Alternatively, could we use the _PAGE_GNTTAB PTE flag that is used for debugging? Indeed, if we did this, could be obviate the need for the PTE-zapping hook, by instead catching the case where this flag is set, and unmapping the grant implicitly?

Otherwise, what would the semantics of this new flag be?

Regards,

Derek.

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