Re: [PATCH v3 04/12] Add "handle page fault" PV helper.

From: Gleb Natapov
Date: Fri Jan 22 2010 - 02:25:43 EST

On Thu, Jan 21, 2010 at 07:47:22AM -0800, H. Peter Anvin wrote:
> On 01/21/2010 01:02 AM, Avi Kivity wrote:
> >>
> >> You can also just emulate the state transition -- since you know
> >> you're dealing with a flat protected-mode or long-mode OS (and just
> >> make that a condition of enabling the feature) you don't have to deal
> >> with all the strange combinations of directions that an unrestricted
> >> x86 event can take. Since it's an exception, it is unconditional.
> >
> > Do you mean create the stack frame manually? I'd really like to avoid
> > that for many reasons, one of which is performance (need to do all the
> > virt-to-phys walks manually), the other is that we're certain to end up
> > with something horribly underspecified. I'd really like to keep as
> > close as possible to the hardware. For the alternative approach, see Xen.
> >
> I obviously didn't mean to do something which didn't look like a
> hardware-delivered exception. That by itself provides a tight spec.
> The performance issue is real, of course.
> Obviously, the design of VT-x was before my time at Intel, so I'm not
> familiar with why the tradeoffs that were done they way they were.
Is it so out of question to reserver exception below 32 for PV use?

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