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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: Gerd Hoffmann <kraxel@xxxxxxxxxx>
Date: Wed, 05 Dec 2007 21:06:08 +0100
Cc: Derek Murray <Derek.Murray@xxxxxxxxxxxx>, "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>
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>> 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?
> 
> Well, in the general case you don't have enough info to know which grant to
> release (a single page can be granted multiple times).

You'll also get the mm and the addr which should make it sufficiently
unique, so this looks like a doable approach to me.

ptep_get_and_clear_full() in include/asm-x86/pgtable_32.h needs to be
changed take care, but that should be possible to do and the change is
local to x86 paravirt_ops, which looks much better to me than touching
generic mm code.

cheers,
  Gerd



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