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

From: Boris Ostrovsky
Date: Wed Nov 18 2015 - 09:03:54 EST


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.


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.

Yes, HVMlite (or however we will end up calling it) will allow these sorts of thing. But HVMlite is not supported yet at all. And it's x86 only.

-boris


else
irq = irq_alloc_desc_at(gsi, -1);
@@ -445,8 +445,8 @@ static void xen_free_irq(unsigned irq)

kfree(info);

- /* Legacy IRQ descriptors are managed by the arch. */
- if (irq < NR_IRQS_LEGACY)
+ /* On HVM legacy IRQ descriptors are managed by the arch. */
+ if (xen_hvm_domain() && irq < NR_IRQS_LEGACY)
return;

irq_free_desc(irq);

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