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Re: [Xen-devel] Design question for PV superpage support

To: Mick.Jordan@xxxxxxx
Subject: Re: [Xen-devel] Design question for PV superpage support
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Tue, 03 Mar 2009 09:26:16 -0800
Cc: Dave McCracken <dcm@xxxxxxxx>, Xen Developers List <xen-devel@xxxxxxxxxxxxxxxxxxx>
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Mick Jordan wrote:
Since restore already requires quite a lot of reset, e.g., grant table mappings, on the part of the guest, it seems that checking the validity of any large page mappings should be possible at the same time. Obviously you could get in a big mess if you mapped the code that is going to do the fixup on a large page, but that is unlikely and easily avoidable.

That's actually the most likely case in Linux. Not being able to use 2M mappings for kernel code+data removes about 95% of the utility.

In general, I think the guest should assume that large page mappings are merely an optimization that (a) might not be possible on domain start due to machine memory fragmentation and (b) that this condition might also occur on restore. Given these, it must always be prepared to function with 4K pages, which implies that it would need to preserve enough page table frame memory to be able revert from large to small pages.

I think that too intrusive. I'd want to see some very convincing measurements to justify doing these kinds of changes to pvops Linux, for example.


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