Re: [patch 0/6] Guest page hinting version 6.

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Tue, 2008-05-06 at 20:49 -0700, Zachary Amsden wrote:
> > I suspect one of the problems is that there are too many state transitions
> > to have it implemented with a low overhead on anything but S390, and even
> > there you need milicoded instructions to handle things.
> > 
> > If the number of transitions can be reduced, page hinting could be useful
> > for KVM, too.
> 
> Spot on Rik, if every transition becomes a hypercall (and a synchronous
> one at that), it isn't workable for us.  If, on the other hand, you
> share the state bits between the guest and hypervisor, you need a giant
> (standalone) bit array for per-page state, which is neither convenient
> for Linux nor the hypervisor.  I believe s390 has an 'instruction' to
> migrate the state bits into the hypervisor per-physical-page data
> without requiring a hypercall.

That is why we invented the millicoded ESSA instruction on s390. We had
an emulation of the instruction to test things. It worked but was
awfully slow.

-- 
blue skies,
  Martin.

"Reality continues to ruin my life." - Calvin.