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Re: [Xen-devel] Error restoring DomU when using GPLPV

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>, ANNIE LI <annie.li@xxxxxxxxxx>
Subject: Re: [Xen-devel] Error restoring DomU when using GPLPV
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Thu, 20 Aug 2009 11:19:37 +0100
Cc: Joshua West <jwest@xxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On 20/08/2009 10:42, "James Harper" <james.harper@xxxxxxxxxxxxxxxx> wrote:

>> Ultimately the 'right' fix will need to be implemented in Xen and dom0
>> tools. But the above kludge would work perfectly well I believe.
> Kludgy, as you say, but if it works then so be it.
> Is there a future proof way for the drivers to know if they are running
> under a future version of xen that isn't broken in this way? I'm
> guessing not...

No, not really. We could add a xenstore flag or something I suppose. But
really losing a few memory pages is not the end of the world. I suppose the
major pain might be if it shatters a physical superpage, and hence makes
VT-d/EPT type stuff more expensive. If you could at least arrange for the
pages to come from the same aligned 2MB region, or even from the bottom 2MB
of memory (which can never be allocated as a superpage because of the VGA
area), that might be nice.

Thinking about how to fix this nicely in the tools, it seems pretty tricky
if I don't want to have to change the dom0 kernel too. The kernel is quite
involved in mapping foreign pages up to user space and gets in the way of
hacking in a flag between tools and Xen... And we do want to be able to map
foreign Xen-heap pages in some cases. It's only a nuisance for

 -- Keir

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