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Re: [Xen-devel] [rfc 00/18] ioemu: use devfn instead of slots as the uni

To: Simon Horman <horms@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] [rfc 00/18] ioemu: use devfn instead of slots as the unit for passthrough
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Thu, 05 Mar 2009 09:22:28 +0000
Cc: Yuji Shimada <shimada-yxb@xxxxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
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On 05/03/2009 09:05, "Simon Horman" <horms@xxxxxxxxxxxx> wrote:

> * pass-through devices may not share a GSI with another pass-through
>   device, and its better if they don't share a GSI with an ioemu
>   device either

Why is such sharing disallowed? Is this a problem with mapping multiple MSI
sources to a single level-triggered GSI? If we can reliably map one MSI to
an emulated GSI, I would have thought that wire-ORing them would be easy,
but perhaps the emulation is dodgy to begin with, even without sharing?

> I can't actually get a system with this mapping to boot
> (which prehaps means its flawed for the first few devices).
> But a fundamental problem (e.g. for users) with using
> such an allocation stratergy?

Yes, a sparse mapping into the devfn space is absolutely fine. Indeed you'll
be needing to consider all that space to make efficient use of the currently
available 32 GSIs.

 -- Keir

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