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

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Message-ID: <20100121090421.GS5238@redhat.com>
Date:	Thu, 21 Jan 2010 11:04:22 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>, kvm@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, mingo@...e.hu,
	tglx@...utronix.de, riel@...hat.com, cl@...ux-foundation.org
Subject: Re: [PATCH v3 04/12] Add "handle page fault" PV helper.

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 23.2.1.3.
> >>
> >
> >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.

--
			Gleb.
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