Re: [Xen-devel] [PATCH v9 3/3] tools/libxc: use superpages during restore of HVM guest

[Olaf Hering <olaf@xxxxxxxxx>]

On Wed, Sep 06, Andrew Cooper wrote:

> I still fail to understand why you need the bitmaps at all?  You can
> calculate everything you need from the pfn list alone, which will also
> let you spot the presence or absence of the VGA hole.

These bitmaps track if a range has been allocated as superpage or not.
If there is a given pfn within a range of either 1G or 2M there might be
double allocation of a 1G or 2M page. This is not related to the VGA
hole. These two lines are just hints that in this range no superpage can
be allocated.

> You need to track which pfns you've see so far in the stream, and which
> pfns have been populated.  When you find holes in the pfns in the
> stream, you need to undo the prospective superpage allocation.  Unless
> I've missed something?

This is whats happening, holes will be created as soon as they are seen
in the stream.

> Also, please take care to use 2M decrease reservations wherever
> possible, or you will end up shattering the host superpage as part of
> trying to remove the memory.

This is what Wei suggested, build a list of pfns instead of releasing
each pfn individually. I think with this new code it should be possible
to decrease in 2M steps as needed.

Olaf

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