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RE: [Xen-devel] [RFC] Pass-through Interdomain Interrupts Sharing (HVM/D

To: "Keir Fraser" <keir@xxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] [RFC] Pass-through Interdomain Interrupts Sharing (HVM/Dom0)
From: "Guy Zana" <guy@xxxxxxxxxxxx>
Date: Fri, 10 Aug 2007 11:51:18 -0400
Cc: Alex Novik <alex@xxxxxxxxxxxx>
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Thread-topic: [Xen-devel] [RFC] Pass-through Interdomain Interrupts Sharing (HVM/Dom0)
> -----Original Message-----
> From: Keir Fraser [mailto:keir@xxxxxxxxxxxxx] 
> Sent: Friday, August 10, 2007 4:18 PM
> To: Guy Zana; xen-devel@xxxxxxxxxxxxxxxxxxx
> Cc: Alex Novik
> Subject: Re: [Xen-devel] [RFC] Pass-through Interdomain 
> Interrupts Sharing (HVM/Dom0)
> On 10/8/07 12:50, "Guy Zana" <guy@xxxxxxxxxxxx> wrote:
> >> It would cycle through the priority list, moving frontmost 
> to back at 
> >> each stage, until the line is deasserted.
> > 
> > 1. When will you deassert the HVM vline?
> I would turn vline assertions into pulses: the line would be 
> asserted only instantaneously, to get latched by the 
> VPIC/VIOAPIC. Actually I think this question is quite 
> separate from whatever method we use for interrupt
> sharing: when would you deassert the vline when the interrupt 
> is *not* shared? Whatever method we choose should be 
> extendable to the shared case, and applied to whichever HVM 
> guest we are currently choosing to deliver the interrupt to. 
> So, whether the interrupt is shared or not, I see no value in 
> modelling the state of the level-triggered vline.

Sounds good actually :-)

> > 2. How do you avoid HVM spurious interrupts?
> I avoid most of them by the fact that a HVM guest that is not 
> handling interrupts will get pushed down the priority list. 
> Of course this won't get rid of all spurious interrupts, but 
> I'd expect it to get rid of enough (e.g., at least 50% even 
> in some worst cases I can think of). So the question is: how 
> sensitive is Windows to spurious interrupts? I know that 
> Linux needs something like 99% of interrupts to be spurious 
> for it to generate a warning. If Windows is similar then my 
> approach would work just fine.

>From what I saw, Windows XP is not that sensitive to spurious interrupts (at 
>least for ISA interrupts). In general, Windows tries hard to survive :-)
We'll have to check if a prioritize list will suffice, it would be simple, I 
But you still do bad stuff and hope it'll go unnoticed, sounds like a recipe 
for voodoo, it should be well tested at least.


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