Re: [Xen-devel] [PATCH RFC] libxl: set 1GB MMIO hole for PVH

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, May 09, 2018 at 06:12:28PM +0200, Juergen Gross wrote:
> On 09/05/18 18:07, Roger Pau Monne wrote:
> > This prevents page-shattering, by being able to populate the RAM
> > regions below 4GB using 1GB pages, provided the guest memory size is
> > set to a multiple of a GB.
> > 
> > Note that there are some special and ACPI pages in the MMIO hole that
> > will be populated using smaller order pages, but those shouldn't be
> > accessed as often as RAM regions.
> 
> Would it be possible somehow to put a potential firmware into that
> 1GB region, too, if it needs any memory in high memory? Seabios e.g.
> is taking the last RAM page of the guest for its hypercall page, which
> will again shatter GB mappings.

I know this comment is related to HVM guests, but I'm not sure I see
how setting the hypercall page shatters GB mappings. Setting the
hypercall page doesn't involve changing any p2m mappings, but just
filling a guest RAM page with some data.

Roger.

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