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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 19/11/07 20:27, "Stephen C. Tweedie" <sct@xxxxxxxxxx> wrote:

> 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.

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.

 -- Keir



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