Re: [PATCH v4] KVM: Allow host IRQ sharing for assigned PCI 2.3 devices

[Avi Kivity <avi@redhat.com>]

On 03/06/2012 05:41 PM, Jan Kiszka wrote:
> On 2012-03-06 16:34, Avi Kivity wrote:
> > On 02/28/2012 03:19 PM, Jan Kiszka wrote:
> >> PCI 2.3 allows to generically disable IRQ sources at device level. This
> >> enables us to share legacy IRQs of such devices with other host devices
> >> when passing them to a guest.
> >>
> >> The new IRQ sharing feature introduced here is optional, user space has
> >> to request it explicitly. Moreover, user space can inform us about its
> >> view of PCI_COMMAND_INTX_DISABLE so that we can avoid unmasking the
> >> interrupt and signaling it if the guest masked it via the virtualized
> >> PCI config space.
> > 
> > Long delay, sorry.
> > 
> > I'm sure we discussed this before, so a URL would be sufficient: why
> > cannot this be transparent to userspace?
>
> Yes, we did, and you may recall I tried hard to implement it. The
> reasons for not following this path were given in one of the previous
> postings:
>
> "To recall the history of it: I tried hard to implement an adaptive
> solution that automatically picks the fastest masking technique whenever
> possible. However, the changes required to the IRQ core subsystem and
> the logic of the device assignment code became so complex and partly
> ugly that I gave up on this. It's simply not worth the pain given that
> legacy PCI interrupts are rarely raised for performance critical device
> at such a high rate (KHz...) that you can measure the difference."

Ok.  Given the paragraph below I won't try to second-guess you.

> > 
> > As for the actual patch, I am so unfamiliar with the device assignment
> > code now that I'll have to rely on Alex's review.
> > 
>
> Then let's hope he didn't miss any of my bugs.
>
> Jan
>


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