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Re: [Xen-devel] [VTD][patch 0/5] HVM device assignment using vt-d

To: "Kay, Allen M" <allen.m.kay@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [VTD][patch 0/5] HVM device assignment using vt-d
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Wed, 30 May 2007 20:55:57 +0100
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On 30/5/07 20:05, "Kay, Allen M" <allen.m.kay@xxxxxxxxx> wrote:

>     - grub.conf: use "ioapic_ack=old" for /boot/xen.gz
>       (io_apic.c contains code for avoiding global interrupt problem)

How does this new scheme work? Can it supplant the ioapic_ack=new method?
Clearly requiring the use of a hacky command-line option to make use of a
new core feature is not very nice to say the least.

It looks like the interrupt gets EOIed by writing to the IOSAPIC EOI
register. I thought that x86 IOAPICs don't have that register?

>   - Revert back to ioapic_ack=old to allow for IRQ sharing amongst
> guests.

I would expect it to work (by design at least) even with ioapic_ack=new.

Actually I also know there are some other patches coming down the pipeline
to do pci passthrough to HVM guests without need for hardware support (of
course it is not so general; in particular it will only work for one special
hvm guest). However, they deal with this interrupt issue quite cunningly, by
inverting the interrupt polarity so that they get interrupts on both +ve and
-ve edges of the INTx line. This allows the virtual interrupt wire to be
'wiggled' precisely according to the behaviour of the physical interrupt
wire. Which is rather nice, although of course it does double the interrupt
rate, which is not so great but perhaps acceptable for the kind of low
interrupt rate devices that most people would want to hand off to a hvm

 -- Keir

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