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

From: Gleb Natapov
Date: Thu Jan 21 2010 - 04:05:19 EST

On Thu, Jan 21, 2010 at 11:02:19AM +0200, Avi Kivity wrote:
> On 01/20/2010 07:43 PM, H. Peter Anvin wrote:
> >On 01/20/2010 02:02 AM, Gleb Natapov wrote:
> >>
> >>>You can have the guest OS take an exception on a vector above 31 just
> >>>fine; you just need it to tell the hypervisor which vector it, the OS,
> >>>assigned for this purpose.
> >>>
> >>VMX doesn't allow to inject hardware exception with vector
> >>greater then 31.
> >>SDM 3B section
> >>
> >
> >OK, you're right. I had missed that... I presume it was done for
> >implementation reasons.
> My expectation is that is was done for forward compatibility reasons.
> >
> >>I can inject the event as HW interrupt on vector greater then 32 but not
> >>go through APIC so EOI will not be required. This sounds
> >>non-architectural
> >>and I am not sure kernel has entry point code for this kind of event, it
> >>has one for exception and one for interrupts that goes through
> >>__do_IRQ()
> >>which assumes that interrupts should be ACKed.
> >
> >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.
That and our event injection path can't play with guest memory right now
since it is done from atomic context.

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