Re: [Xen-devel] [PATCH] xen/events: Always allocate legacy interrupts on PV guests

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 11/18/2015 09:28 AM, Vitaly Kuznetsov wrote:
> Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> writes:
>
> > On 11/18/2015 06:16 AM, Vitaly Kuznetsov wrote:
> > > Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> writes:
> > >
> > > > After commit 8c058b0b9c34 ("x86/irq: Probe for PIC presence before
> > > > allocating descs for legacy IRQs") early_irq_init() will no longer
> > > > preallocate descriptors for legacy interrupts if PIT does not
> > > > exist.
> > > PIC?
> > Right. David, can you fix this before committing?
> >
> > > > Therefore we need to allocate those descriptors for PV guests
> > > > ourselves.
> > > >
> > > > Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > > > Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > > ---
> > > >    drivers/xen/events/events_base.c | 8 ++++----
> > > >    1 file changed, 4 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/drivers/xen/events/events_base.c b/drivers/xen/events/events_base.c
> > > > index 849500e..a2bb333 100644
> > > > --- a/drivers/xen/events/events_base.c
> > > > +++ b/drivers/xen/events/events_base.c
> > > > @@ -419,8 +419,8 @@ static int __must_check xen_allocate_irq_gsi(unsigned gsi)
> > > >         if (xen_pv_domain() && !xen_initial_domain())
> > > >                 return xen_allocate_irq_dynamic();
> > > >
> > > > -       /* Legacy IRQ descriptors are already allocated by the arch. */
> > > > -       if (gsi < NR_IRQS_LEGACY)
> > > > +       /* On HVM legacy IRQ descriptors are already allocated by the arch. */
> > > > +       if (xen_hvm_domain() && gsi < NR_IRQS_LEGACY)
> > > >                 irq = gsi;
> > > Wouldn't it be better to write it as
> > >    if (gsi < nr_legacy_irqs()) ?
> > I don't think so: on PV we end up setting legacy_pic to
> > null_legacy_pic in probe_8259A() and that will make nr_legacy_irqs()
> > return 0.
> Yes, so the condition will always be false for PV and it equals to
> xen_hvm_domain() or am I missng something?
Oh, I see what you are saying. Yes, it would be cleaner.

-boris



> > > I think it's possible to have PIC-less HVM guests in future (btw, what
> > > about HVMlite?). I see nr_legacy_irqs() is x86-only but it can easily be
> > > defined to NR_IRQS_LEGACY on other arches.

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