Re: Design question for PV superpage support

[Jeremy Fitzhardinge <jeremy@goop.org>]

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