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Re: [Xen-devel] Re: [PATCH] xen: fix interrupt routing

To: Avi Kivity <avi@xxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH] xen: fix interrupt routing
From: Alexander Graf <agraf@xxxxxxx>
Date: Wed, 15 Jun 2011 18:54:30 +0200
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>, Jan Kiszka <jan.kiszka@xxxxxxxxxxx>, "qemu-devel@xxxxxxxxxx" <qemu-devel@xxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>, Isaku Yamahata <yamahata@xxxxxxxxxxxxx>, Anthony Liguori <anthony@xxxxxxxxxxxxx>
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Am 15.06.2011 um 18:34 schrieb Avi Kivity <avi@xxxxxxxxxx>:

> On 06/15/2011 11:24 AM, Alexander Graf wrote:
>> I'm actually not quite sure what exactly he's describing here. But if it's 
>> bypassing the bus logic, it's not a normal PCI device :). Sure, there are 
>> special case devices that also expose a PCI interface. But real PCI cards 
>> that you plug in onto the PCI bus can't bypass the interrupt logic of the 
>> bus, as the only interrupt wires they have go to the bus. And since the PCI 
>> adapters we use in PC machines in Qemu are all non-special, guests can 
>> possibly choke on this.
> There actually is a special device in qemu - acpi power management is 
> configured as a PCI device, but its interrupt is hard-wired to gsi 9 and is 
> edge-triggered (so it can't share the irq line).
> I other devices that are special in this regard to also be part of the 
> chipset, not devices you can plug into arbitrary slots.

Sure, platform devices can do that. Real PCI cards can not. Have you ever seen 
an e1000 with direct mapped interrupt lines? :)

I admit though that we also emulate platform devices that happen to expose 
themselves on the PCI bus. It's not common though and I wouldn't expect every 
OS/driver to be happy about it.



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