Re: [Xen-devel] [PATCH v9 07/10] xen: remove workaround to inject evtchn_irq on irq enable

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 4 Aug 2014, Jan Beulich wrote:
> >>> On 01.08.14 at 19:00, <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > On Fri, 25 Jul 2014, Jan Beulich wrote:
> >> but I still don't see why ARM needs what x86 (even for HVM) appears to
> >> get along fine without.
> > 
> > Good question.
> > x86 PV guests have in xen_irq_enable:
> > 
> > if (unlikely(vcpu->evtchn_upcall_pending))
> >         xen_force_evtchn_callback();
> > 
> > Also xen_irq_enable_direct calls check_events.
> > 
> > I suspect that PV on HVM guests that get events via gsi interrupts get
> > away without it because they only call VCPUOP_register_vcpu_info on
> > secondary cpus.
> 
> But my pointer was specifically to pure HVM guests (among all the
> other possible kinds)...

Pure HVM guests don't map the vcpu info struct so they don't have this
problem. After all they don't have event channels.
More advanced forms of PV on HVM guests have vector callbacks that need
no emulation at the vioapic/vlapic level.
So that leaves us with old style PV on HVM guests, that receive event
notifications via legacy interrupts, but only to the first vcpu. This is
the case I am talking about.

Am I missing anything?

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