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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>, <keir@xxxxxxxxxxxxx>, <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 11:19:51 +0100
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On 28/5/07 10:42, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:

>> The new physdev_irq_permission can also provide a translation service
>> into a
>> new per-domain pirq space. Takes parameters something like:
>> @type: MSI, or INTx
>> @idx: vector (MSI type), or irq number as specified to alloc_irq_vector
>> (INTx type)
>> @domain: domain to remap to
>> @pirq: 'pirq' in that domain (allow caller to specify, or -1 to
>> auto-allocate).
> Who is the very owner to allocate per-domain pirq, domain itself or Xen?
> Or do you mean both sides can allocate by either "caller to specify "
> or '-1' for auto-allocation?

I mean the caller can decide whether he allocates or whether Xen allocates.
My thinking is that if we made it so that this command needed to be used to
'wire up' MSIs into dom0's pirq namespace, and we're already implicitly
mapping PHYSDEVOP_alloc_irq_vector()-allocated IRQs into dom0's pirq
namespace, it may be that dom0 has a region of its irq namespace that it
would like to use for MSIs and, if so, it is best placed to specify the pirq
parameter for itself. But I would expect that in many/most cases we'd be
happy to leave the allocation to Xen.

If we wanted to just support one or the other doing the allocation, I would
say we should leave it to Xen, and make pirq an output-only field (rather
than in/out, as I specified it).

 -- Keir

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