Re: [PATCH v9 1/2] kvm: Use a reserved IRQ source ID for irqfd

Avi Kivity
2012-09-05T15:15:08Z

On 09/05/2012 06:07 PM, Michael S. Tsirkin wrote:
> On Wed, Sep 05, 2012 at 05:46:17PM +0300, Avi Kivity wrote:
>> On 08/21/2012 10:29 PM, Alex Williamson wrote:
>> > KVM_IRQFD currently uses the reserved KVM_USERSPACE_IRQ_SOURCE_ID
>> > which is also shared with userspace injection methods like
>> > KVM_IRQ_LINE. This can cause a conflict if an irqfd triggers on
>> > a GSI asserted through KVM_IRQ_LINE. Move irqfd to it's own
>> > reserved IRQ source ID. Add a capability for userspace to test
>> > for this fix.
>>
>> I don't think we need a cap, rather a backport if we identify real cases
>> where an edge gsi is shared among several devices. Otherwise it is just
>> a theoretical bug before level irqfd is introduced.
>
> In that case, I think it's safer to preserve the "bug" as is: we are
> changing userspace-visible behaviour for edge interrupts otherwise.
> For example if userspace uses kvm_irq_line for an edge
> interrupt, set it to 1, previously it could then
> continue to send any number of interrupts with irqfd,
> now it can't.

If anyone did that, they should have reported a bug, since they surely
didn't expect edges if the line was held high.

>
> Basically the logical OR functionality of source IDs
> does not make sense for edge.

Edge is only interpreted at the ioapic or pic input; the line is just a
line (an open collector line that ORs anything connected to it, or an
equivalent).

>
> How about we do
> if (flags&RESAMPLE)
> source_id = USERSPACE
> else
> source_id = IRQFD

Okay if we identify something that depends on the bug, otherwise not.

--
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