Re: [Qemu-devel] [RFC][PATCH 12/45] msi: Introduce MSIRoutingCache

[Jan Kiszka]

On 2011-10-17 17:43, Michael S. Tsirkin wrote:
> On Mon, Oct 17, 2011 at 11:27:46AM +0200, Jan Kiszka wrote:
>> This cache will help us implementing KVM in-kernel irqchip support
>> without spreading hooks all over the place.
>>
>> KVM requires us to register it first and then deliver it by raising a
>> pseudo IRQ line returned on registration. While this could be changed
>> for QEMU-originated MSI messages by adding direct MSI injection, we will
>> still need this translation for irqfd-originated messages. The
>> MSIRoutingCache will allow to track those registrations and update them
>> lazily before the actual delivery. This avoid having to track MSI
>> vectors at device level (like qemu-kvm currently does).
>>
>> Signed-off-by: Jan Kiszka <address@hidden>
> 
> So if many devices are added, exhausting the number of GSIs supported,
> we get terrible performance intead of simply failing outright.
> 
> To me, this looks more like a bug than a feature ...

If that ever turns out to be a bottleneck, failing looks like the worst
we can do. Reporting excessive cache flushes would make some sense and
could still be added.

Jan

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