Re: [PATCH] KVM: Use IRQF_ONESHOT for assigned device MSI interrupts

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-06-08 10:00, Michael S. Tsirkin wrote:
> On Fri, Jun 08, 2012 at 09:55:01AM +0200, Jan Kiszka wrote:
>> On 2012-06-08 09:47, Michael S. Tsirkin wrote:
>>> On Mon, Jun 04, 2012 at 01:40:28PM +0200, Jan Kiszka wrote:
>>>> On 2012-06-04 13:21, Thomas Gleixner wrote:
>>>>> On Sun, 3 Jun 2012, Avi Kivity wrote:
>>>>>
>>>>>> On 06/01/2012 09:26 PM, Jan Kiszka wrote:
>>>>>>>
>>>>>>>> you suggesting we need a request_edge_threaded_only_irq() API?  Thanks,
>>>>>>>
>>>>>>> I'm just wondering if that restriction for threaded IRQs is really
>>>>>>> necessary for all use cases we have. Threaded MSIs do not appear to me
>>>>>>> like have to be handled that conservatively, but maybe I'm missing some
>>>>>>> detail.
>>>>>>>
>>>>>>
>>>>>> btw, I'm hoping we can unthread assigned MSIs.  If the delivery is
>>>>>> unicast, we can precalculate everything and all the handler has to do is
>>>>>> set the IRR, KVM_REQ_EVENT, and kick the vcpu.  All of these can be done
>>>>>> from interrupt context with just RCU locking.
>>>>>
>>>>> There is really no need to run MSI/MSI-X interrupts threaded for
>>>>> KVM. I'm running the patch below for quite some time and it works like
>>>>> a charm.
>>>>>
>>>>> Thanks,
>>>>>
>>>>> 	tglx
>>>>> ----
>>>>> Index: linux-2.6/virt/kvm/assigned-dev.c
>>>>> ===================================================================
>>>>> --- linux-2.6.orig/virt/kvm/assigned-dev.c
>>>>> +++ linux-2.6/virt/kvm/assigned-dev.c
>>>>> @@ -105,7 +105,7 @@ static irqreturn_t kvm_assigned_dev_thre
>>>>>  }
>>>>>  
>>>>>  #ifdef __KVM_HAVE_MSI
>>>>> -static irqreturn_t kvm_assigned_dev_thread_msi(int irq, void *dev_id)
>>>>> +static irqreturn_t kvm_assigned_dev_msi_handler(int irq, void *dev_id)
>>>>>  {
>>>>>  	struct kvm_assigned_dev_kernel *assigned_dev = dev_id;
>>>>>  
>>>>> @@ -117,7 +117,7 @@ static irqreturn_t kvm_assigned_dev_thre
>>>>>  #endif
>>>>>  
>>>>>  #ifdef __KVM_HAVE_MSIX
>>>>> -static irqreturn_t kvm_assigned_dev_thread_msix(int irq, void *dev_id)
>>>>> +static irqreturn_t kvm_assigned_dev_msix_handler(int irq, void *dev_id)
>>>>>  {
>>>>>  	struct kvm_assigned_dev_kernel *assigned_dev = dev_id;
>>>>>  	int index = find_index_from_host_irq(assigned_dev, irq);
>>>>> @@ -346,9 +346,8 @@ static int assigned_device_enable_host_m
>>>>>  	}
>>>>>  
>>>>>  	dev->host_irq = dev->dev->irq;
>>>>> -	if (request_threaded_irq(dev->host_irq, NULL,
>>>>> -				 kvm_assigned_dev_thread_msi, 0,
>>>>> -				 dev->irq_name, dev)) {
>>>>> +	if (request_irq(dev->host_irq, kvm_assigned_dev_msi_handler, 0,
>>>>> +			dev->irq_name, dev)) {
>>>>>  		pci_disable_msi(dev->dev);
>>>>>  		return -EIO;
>>>>>  	}
>>>>> @@ -373,9 +372,9 @@ static int assigned_device_enable_host_m
>>>>>  		return r;
>>>>>  
>>>>>  	for (i = 0; i < dev->entries_nr; i++) {
>>>>> -		r = request_threaded_irq(dev->host_msix_entries[i].vector,
>>>>> -					 NULL, kvm_assigned_dev_thread_msix,
>>>>> -					 0, dev->irq_name, dev);
>>>>> +		r = request_irq(dev->host_msix_entries[i].vector,
>>>>> +				kvm_assigned_dev_msix_handler, 0,
>>>>> +				dev->irq_name, dev);
>>>>>  		if (r)
>>>>>  			goto err;
>>>>>  	}
>>>>
>>>> This may work in practice but has two conceptual problems:
>>>>  - we do not want to run a potential broadcast to all VCPUs to run in
>>>>    a host IRQ handler
>>>>  - crazy user space could have configured the route to end up in the
>>>>    PIC or IOAPIC, and both are not hard-IRQ safe (this should probably
>>>>    be caught on setup)
>>>>
>>>> So this shortcut requires some checks before being applied to a specific
>>>> MSI/MSI-X vector.
>>>
>>> I did this in the past:
>>> https://lkml.org/lkml/2012/1/18/287
>>>
>>> Have no hw to test this atm but if there are any takers
>>> wanting to play with it I can update and post.
>>
>> Just add check that allow only unicasts, and this should be fine.
>>
>> Jan
> 
> If I code it up you can test it?

Yep, no problem.

Jan

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