Re: [PATCH v8 0/6] kvm: level irqfd support

Avi Kivity
2012-08-16T18:22:15Z

On 08/15/2012 10:22 PM, Michael S. Tsirkin wrote:
> On Wed, Aug 15, 2012 at 11:36:31AM -0600, Alex Williamson wrote:
>> On Wed, 2012-08-15 at 17:28 +0300, Michael S. Tsirkin wrote:
>> > On Fri, Aug 10, 2012 at 04:37:08PM -0600, Alex Williamson wrote:
>> > > v8:
>> > >
>> > > Trying a new approach. Nobody seems to like the internal IRQ
>> > > source ID object and the interactions it implies between irqfd
>> > > and eoifd, so let's get rid of it. Instead, simply expose
>> > > IRQ source IDs to userspace. This lets the user be in charge
>> > > of freeing them or hanging onto a source ID for later use.
>> >
>> > In the end it turns out source ID is an optimization for shared
>> > interrupts, isn't it? Can't we apply the optimization transparently to
>> > the user? E.g. if we have some spare source IDs, allocate them, if we
>> > run out, use a shared source ID?
>>
>> Let's think about shared source IDs a bit more. I think it's wrong that
>> irqfd uses KVM_USERSPACE_IRQ_SOURCE_ID, but I'm questioning whether all
>> irqfd users can share a source ID. We do not get the logical OR of all
>> users by putting them on the same source ID, we get "last set wins".
>> KVM_USERSPACE_IRQ_SOURCE_ID is used for multiple inputs because the
>> logical OR happens in userspace. How would we not starve a user if we
>> define KVM_IRQFD_SOURCE_ID? What am I missing?
>
> That all irqfds are deasserted on EOI anyway. So there's no point
> to do a logical OR.
>
>

What if a level irqfd shares a line with a KVM_IRQ_LINE ioctl? Then an
EOI can de-assert the irqfd source, but the line is kept high by the
last KVM_IRQ_LINE invocation.

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