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

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Message-ID: <564C8531.2000608@oracle.com>
Date:	Wed, 18 Nov 2015 09:03:29 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>, david.vrabel@...rix.com
Cc:	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 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.

>
> 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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