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

["Ian Pratt" <Ian.Pratt@xxxxxxxxxxxxx>]

> 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
get
> it, then that is a net lose for the guest --- the guest takes all of
the
> 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
sure
> 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
assumption.

Ian


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