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

["Michael S. Tsirkin" <mst@redhat.com>]

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.

-- 
mst