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