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RE: [Xen-devel] [RFC][PATCH]Large Page Support for HAP

> On Tue, 2007-11-20 at 10:27 +0000, Keir Fraser wrote:
> > An HVM guest always thinks it has big contiguous chunks of RAM. The
> > superpage mappings get shattered invisibly by the HV in the shadow
> page
> > tables only if 2M/4M allocations were not actually possible. This
> shattering
> > happens unconditionally right now, so what's being proposed is a net
> benefit
> > to HVM guests.
> If an HVM guest asks for a bigpage allocation and silently fails to
> it, then that is a net lose for the guest --- the guest takes all of
> pain for none of the benefits of bigpage.
> So, you may be better off not offering bigpages at all than offering
> them on a best-effort basis; at least that way the guest knows for
> what resources it has available.

Unfortunately, a number of guests assume big pages without actually
checking for the feature bit explicitly. For example x86_64 Linux
running HVM will assume it has big pages. We're able to hack this
assumption out of it in PV mode.  IIRC Windows makes the same big page


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