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

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Date:	Fri, 8 Jun 2012 11:00:59 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Avi Kivity <avi@...hat.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"yongjie.ren@...el.com" <yongjie.ren@...el.com>
Subject: Re: [PATCH] KVM: Use IRQF_ONESHOT for assigned device MSI interrupts

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?

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> Corporate Competence Center Embedded Linux
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