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

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Date:	Wed, 18 Nov 2015 10:00:35 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.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

On 11/18/2015 09:28 AM, Vitaly Kuznetsov wrote:
> 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?

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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