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

From: Boris Ostrovsky
Date: Wed Nov 18 2015 - 10:01:08 EST

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
Right. David, can you fix this before committing?

Therefore we need to allocate those descriptors for PV guests

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.


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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