Re: [Xen-devel] [PATCH v2 3/3] tools/libxc: use superpages during restore of HVM guest

[Olaf Hering <olaf@xxxxxxxxx>]

On Tue, Aug 22, Wei Liu wrote:

> On Thu, Aug 17, 2017 at 07:01:33PM +0200, Olaf Hering wrote:
> > +    /* No superpage in 1st 2MB due to VGA hole */
> > +    xc_sr_set_bit(0, &ctx->x86_hvm.restore.attempted_1g);
> > +    xc_sr_set_bit(0, &ctx->x86_hvm.restore.attempted_2m);
> I don't quite get this. What about other holes such as MMIO?

This just copies what meminit_hvm does. Is there a way to know where the
MMIO hole is? Maybe I just missed the MMIO part. In the worst case I
think a super page is allocated, which is later split into single pages.

> One potential issue I can see with your algorithm is, if the stream of
> page info contains pages from different super pages, the risk of going
> over memory limit is high (hence failing the migration).
> 
> Is my concern unfounded?

In my testing I have seen the case of over-allocation. Thats why I
implemented the freeing of unpopulated parts. It would be nice to know
how many pages are actually coming. I think this info is not available.

On the other side, the first iteration sends the pfns linear. This is
when the allocation actually happens. So the over-allocation will only
trigger near the end, if a 1G range is allocated but only a few pages
will be stored into this range.

Olaf

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