Re: [Qemu-devel] [RFC][PATCH 00/45] qemu-kvm: MSI layer rework for in-kernel irqchip support

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-10-17 17:57, Michael S. Tsirkin wrote:
> On Mon, Oct 17, 2011 at 11:27:34AM +0200, Jan Kiszka wrote:
>> As previously indicated, I was working for quite a while on a major
>> refactoring of the MSI "additions" we have in qemu-kvm to support
>> in-kernel irqchip, vhost and device assignment. This is now the outcome.
>>
>> I'm quite happy with it, things are still working (apparently), and the
>> invasiveness of KVM hooks into the MSI layer is significantly reduced.
>> Moreover, I was able to port the device assignment code over generic MSI
>> support, reducing the size of that file a bit further.
>>
>> Some further highlights:
>>  - fix for HPET MSI support with in-kernel irqchip
>>  - fully configurable MSI-X (allows 1:1 mapping for assigned devices)
>>  - refactored KVM core API for device assignment and IRQ routing
>>
>> I'm sending the whole series in one chunk so that you can see what the
>> result will be. It's RFC as I bet that there are regressions included
>> and maybe still room left for improvements. Once all is fine (can be
>> broken up into multiple chunks for the merge), I would suggest patching
>> qemu-kvm first and then start with porting things over to upstream.
>>
>> Comments & review welcome.
>>
>> CC: Alexander Graf <agraf@suse.de>
>> CC: Gerd Hoffmann <kraxel@redhat.com>
>> CC: Isaku Yamahata <yamahata@valinux.co.jp>
> 
> 
> So the scheme where we lazily update kvm gsi table
> on interrupts is interesting. There seems to be
> very little point in it for virtio, and it does
> seem to make it impossible to detect lack or resources
> (at the moment we let guest know if we run out of GSIs
> and linux guests can fall back to regular interrupts).

Are we really at that limit already? Then I think it's rather time to
lift it at the kernel side.

> 
> I am guessing the idea is to use it for device assignment
> where it does make sense as there is no standard way
> to track which vectors are actually used?
> But how does it work there? kvm does not
> propage unmapped interrupts from an assigned device to qemu, does it?

No, device assignment and irqfd (vhost, vfio, whatever-will-come) belong
to the static group. They need to hand out a static gsi to the irq
source, thus they cannot participate in lazy updating.

But they can benefit from the generic config notifiers: whenever the
guest changes some route (or disables it), they just need to inform the
core about this. So there is no need to track used vectors separately
anymore.

Jan


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