Re: In kernel PIC support: kernel patch

Gregory Haskins
2007-06-25T22:23:05Z

On Mon, 2007-06-25 at 18:08 -0400, Avi Kivity wrote:
> There's a slight issue that Windows with the ACPI HAL
> is dog slow when virtulalized, but maybe we can find another HAL that
> supports ACPI but doesn't use the APIC.

Is this in reference to the frequent TPR updates, or is there something
else that is also slow with ACPI enabled?

>
> > Note that there is another standard that would allow us to use built-in
> > routing without ACPI/IOAPIC: MSI. I know the battle over
> > virtual-bus-rendering is still raging w.r.t. to PCI or not-PCI. But I
> > will point out that if we do use PCI, setting the PV devices up as MSI
> > capable in the config space potentially eliminates the need to also wire
> > them to an IOAPIC. They will get their vector data from the MSI setup
> > and can then send APIC messages directly. (I think there is some
> > confusion about how we can do MSI later in this thread...I will reply to
> > those mails separately)
> >
>
> MSI is also a good solution; but do all semi-modern guest OSes support it?

I can't say for sure, but I would guess "yes" because IIUC PCI-Express
only uses MSI (or at least heavily prefers it).

> No, a single ioapic in the kernel, with one interrupt line that is
> shared between a kernel-provided device and a userspace-provided
> device. The kernel would need to provide the OR function that sharing
> implies.

Got it. I was confused earlier.

>
>
> > If we do end up needing the IOAPIC in the
> > kernel and userspace, I would suggest using two IOAPICs (we just need to
> > update the ACPI tables to say there are two) so that they can operate
> > independently. This should be supportable from a virtual system
> > infrastructure standpoint, and is much cleaner IMHO. If you were
> > talking about a different scenario. please clarify
>
> I was talking about a different scenario, but your solution is cleaner
> (though it involves more hand-dirtying with the ACPI tables). Basically
> each PCI IRQ would have its own dedicated line.

Right. Or more precisely, *anything* you want can have its own line ;)
IIUC a standard IOAPIC has 24 input lines, so you just need one for
every 24 devices allocated.