Re: [PATCH v2 0/9] qemu-kvm: Clean up and enhance MSI irqchip support

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-04-27 09:27, Avi Kivity wrote:
> On 04/26/2011 04:19 PM, Jan Kiszka wrote:
>> I've still plans to consolidate MSI-X mask notifiers and KVM hooks, but
>> that can wait until we go upstream.
>>
>> This version still makes classic MSI usable in irqchip mode, now not
>> only for PCI devices (AHCI, HDA) but also for the HPET (with msi=on).
>> Moreover, it contains an additional patch to refresh the MSI IRQ routes
>> after vmload.
>>
> 
> Patches 1-8 applied, thanks.  I'm not sure about 9 (hpet kvm msi 
> integration) - it seems very intrusive to do this to every 
> msi-supporting device.  At least for pci we get all pci devices done in 
> one shot.

Right, it is a but intrusive, but I do not see any real alternative.

> 
> We could do this transparently in hw/apic.c.  When the message is sent 
> for the first time we look it up, fail, and update the kvm routing 
> entry.  Next time the lookup succeeds and we just use KVM_IRQ_LINE, 
> until the message changes and we need to update the irq entry again.

I thought about this, also for PCI devices that aren't assigned or
vhost-driven, but we would quickly end up with unused and never freed
IRQ routing entries. We still need to track the vector configurations.

What would help at least in the HPET case is a new DELIVER_MSI syscall
that completely skips the IRQ routing thing. Actually, we only need
routing for IRQs that shall be injected directly at kernel level. OTOH,
this service would not be available on existing kernels, and we would
not be able to simplify the PCI code that way (due to vhost
requirements). So I dropped this idea as well and accepted that IRQ
routing is the way to go.

Jan

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