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

["Stephen C. Tweedie" <sct@xxxxxxxxxx>]

Hi,

On Thu, 2007-11-15 at 10:26 -0600, Huang2, Wei wrote:

> DESIGN IDEAS:
> 1. Large page requests
> - xc_hvm_build.c requests large page (2MB for now) while starting
> guests
> - memory.c handles large page requests. If it can not handle it, falls
> back to 4KB pages.

It makes me uncomfortable if the guest can't be sure that a PSE request
isn't actually being honoured by the hardware.

A guest OS has to go to a lot of trouble to use large pages.  Such pages
upset the normal page recycling of the guest, they are hard to
recycle... but the guest expects that the compromises are worth it
because large pages are more efficient at the hardware level.

So if the HV is only going to supply them on a best-effort basis --- if
a guest cannot actually rely on a large-page request being honoured ---
then it's not clear whether this is a net benefit or a net cost to the
guest.

--Stephen



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