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

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

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.


Taking KVM aside, my general question remains if threaded MSI handlers
of all devices really need to apply IRQF_ONESHOT though they should have
no use for it.

Jan

-- 
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