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

To: Dave McCracken <dcm@xxxxxxxx>
Subject: Re: [Xen-devel] Design question for PV superpage support
From: Mick Jordan <Mick.Jordan@xxxxxxx>
Date: Mon, 02 Mar 2009 10:14:38 -0800
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On 03/02/09 10:00, Dave McCracken wrote:
No, actually, it doesn't do that. The hypervisor allocates 2M pages, then expands them into 4K pages for the mfn<->pfn maps, etc.

The only effective difference is that any given 2M-aligned range of pfns is guaranteed to map to a contiguous 2M-aligned range of mfns. Therefore the guest can safely allocate 2M pages.
Ok. So I want to re-iterate my question from a previous post. After the patch allowing mixed mappings, what exactly went wrong on save/restore. And would my special case of 1-1 physival/virtual mappings with additional 2MB VM mappings adddress after domain start suffer in that case?

From my (brief) experience, I think the problems of finding enough contiguous machine memory to allocate an all 2MB domain might be prohibitive. And when the memory is not fragmented I did not find it hard to "find" contiguous aligned 2MB machine pages even with the usual (seemingly random) pfn -> mfn mappings. It's a bit more code and runtime overhead, but it doesn't happen enough to worry about that.


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