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

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Date:	Wed, 18 Nov 2015 15:28:51 +0100
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	david.vrabel@...rix.com, konrad.wilk@...cle.com, jgross@...e.com,
	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xen.org
Subject: Re: [Xen-devel] [PATCH] xen/events: Always allocate legacy interrupts on PV guests

Boris Ostrovsky <boris.ostrovsky@...cle.com> writes:

> On 11/18/2015 06:16 AM, Vitaly Kuznetsov wrote:
>> Boris Ostrovsky <boris.ostrovsky@...cle.com> 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@...cle.com>
>>> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
>>> ---
>>>   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?

>
>>
>> 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);

-- 
  Vitaly
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