RE: Dynamic-irq's in HVM domains

[Ian Campbell <Ian.Campbell@citrix.com>]

[Inverting your top-post]

> > 'cat /proc/interrupts' in a Linux PV domain shows interrupts starting
> > at 256 and labelled as 'Dynamic-irq'. Are these available in a HVM
> > domain eg under Windows?

On Fri, 2007-11-30 at 13:29 +1100, James Harper wrote:
> I'm really desperate for an answer to this question...

Dynamic IRQs are just a convenient Linux interface used to represent
event channels in order that they can be used with the regular IRQ
subsystem. Although I think they were added by Xen folk I don't think
there's anything Xen specific or special about them.

Event channels are (of course) exposed to HVM guests. The Linux
PV-on-HVM drivers export them itself via their own parallel
interrupt-like subsystem because not all the native versions supported
by the PV-on-HVM drivers have dynamic IRQs. See evtchn_interrupt() and
the other code in unmodified_drivers/linux-2.6/platform-pci/evtchn.c

Perhaps Windows has a similar concept to dynamic IRQs in its IRQ
subsystem, I'm afraid not being a Windows type I have no idea about
that. You can still use event channels outside of the regular windows
IRQ subsystem though, as demonstrated above.

> Failing that, can I use physical IRQ's in a HVM domain to bind to? Eg
> nothing is using IRQ 26, can I use that with bind_pirq? The only docs I
> can find says 'Furthermore the calling domain must be sufficiently
> privileged.', but I'm not sure how privileged 'sufficiently' means in
> this context.

I doubt that will work, I expect privileged means either domain 0 or a
domain with a PCI passthrough device attached...

Ian.