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

[Mick Jordan <Mick.Jordan@xxxxxxx>]

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.
Mick



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