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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 02/03/2009 17:29, "Dave McCracken" <dcm@xxxxxxxx> wrote:

>> This wouldn't work too well for me in the case of thread stacks because
>> we need to map out parts of the stack and, although we want large
>> virtual stacks, we don't want do dedicate that much physical memory. Is
>> it really difficult to support mixed pages sizes in the general case,
>> e.g., save/restore etc.?
> 
> What I am doing is populating the domain with 2M pages.  The hypervisor fills
> in all its internal arrays as if they were regular 4K pages.  The guest is
> then free to use mixed size pages.  The only significant difference is that
> when a guest does allocate a 2M page, it's guaranteed to be properly aligned
> at the machine page level so it can be mapped as a hugepage.   All 4K page
> allocations will continue to work.

It'd be nice to fall back to the case of not being able to guarantee all 2MB
extents are aligned and contiguous. So for example being able to migrate to
or restore on a system that currently doesn't have enough contiguous memory.

 -- Keir



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