Re: [Xen-devel] Xen interrupts when using passthrough

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2011-10-05 at 10:09 +0100, Jan Beulich wrote:
> >>> On 05.10.11 at 11:00, Eli Cohen <eli@xxxxxxxxxxxxxxxxxx> wrote:
> > On Wed, Oct 05, 2011 at 09:51:10AM +0100, Jan Beulich wrote:
> >> Dom0 shouldn't get involved at all - Xen does the necessary conversion.
> >> Dom0 may only be involved in setting things up. Whether they get
> >> received via event channels or virtual interrupts depends on the kind
> >> of guest.
> >> 
> > I think I used the wrong terminology not distinguishing between the
> > hypervisor and dom0 so I will ask again just to make sure. Is the
> > hypervisor involved in passing the interrupts?
> 
> Yes, of course. The hypervisor receives all physical interrupts.
> 
> > Does it depend on whether the guest is paravirtualized or not?
> 
> Yes, at least to some degree.
> 
> > The guest I am using is
> > paravirtualized (i.e uses xen hypercalls), so does it mean that it
> > uses event channels or is there a possibilty that it does not use
> > event channels.
> 
> Yes and no: PV guests can receive interrupts only via the event
> channel mechanism.

While an HVM guest can receive interrupts via any of:
      * Emulated interrupt injection (i.e. from the emulated APIC)
      * Event channels, injected via the INTx interrupt on the PCI
        platform device (itself injected by the previous option)
      * Event channels, injected directly to a vector.

The first is available to any HVM guest, the middle one is the older
style PV Driver for HVM guest mechanism while the third is an additional
feature of more recent Linux kernels using the PVHVM mechanism.

Ian.


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