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Re: [Xen-devel] [PATCH 2/3][RFC] MSI/MSI-X support fordom0/driver domain

To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 2/3][RFC] MSI/MSI-X support fordom0/driver domain
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Mon, 28 May 2007 13:14:10 +0100
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On 28/5/07 13:03, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:

> OK, I agree it's flexible and extensible. But is there any real usage
> model pushing on this? For example, is it better for pciback instance
> to allocate pirq space for domU? Pciback can select whether
> passthrough real irq number or allocate from a new space for
> target domain. To let Xen allocate instead makes it complex.
> Also do we support mixed allocation policy to a given domain,
> when using suggested interface. For example, once one domain
> has BIOS-assigned scheme, it can't request Xen for auto-allocation.
> Or else it's difficult and complex for domain and Xen to sit on same
> page for shared allocation policy. Maybe we need some per-domain
> flag to control such allocation policy?

I think you're making this more complicated than it really is. My preference
for Xen doing the allocation for domUs is only because: what if there are
more places than just pciback that can assign device irqs to domUs? The
implementation complexity in Xen of supporting both allocation methods is
tiny: presumably in either case we will maintain an array of pirqs per
domain (maybe 0-255?) and in the case of Xen doing the allocation it just
has to do a find-first-unused-pirq search. Trivial. Even if it doesn't have
any users in the first instance (because we decide to let domU see real bios
numbers, or whatever), it's a simple service to provide in case it's useful
in future.

 -- Keir

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