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

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Date:	Fri, 01 Jun 2012 11:03:12 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"avi@...hat.com" <avi@...hat.com>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"yongjie.ren@...el.com" <yongjie.ren@...el.com>, tglx@...utronix.de
Subject: Re: [PATCH] KVM: Use IRQF_ONESHOT for assigned device MSI interrupts

On Fri, 2012-06-01 at 18:39 +0200, Jan Kiszka wrote:
> On 2012-06-01 18:16, Alex Williamson wrote:
> > The kernel no longer allows us to pass NULL for a hard interrupt
> > handler without IRQF_ONESHOT.  Should have been using this flag
> > anyway.
> 
> This make the IRQ handling tail a bit slower (due to
> irq_finalize_oneshot). MSIs are edge-triggered, so there was no need for
> masking in theory.

Aren't these asynchronous since we can theoretically do
irq_finalize_oneshot while the guest is servicing the device?

>  Hmm, can't we trust the information that an IRQ
> grabbed here is really a MSI type?


Apparently not, comment added with this check (1c6c6952):

       * The interrupt was requested with handler = NULL, so
       * we use the default primary handler for it. But it
       * does not have the oneshot flag set. In combination
       * with level interrupts this is deadly, because the
       * default primary handler just wakes the thread, then
       * the irq lines is reenabled, but the device still
       * has the level irq asserted. Rinse and repeat....
       *
       * While this works for edge type interrupts, we play
       * it safe and reject unconditionally because we can't
       * say for sure which type this interrupt really
       * has. The type flags are unreliable as the
       * underlying chip implementation can override them.

Thanks,

Alex

> > 
> > Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=43328
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> > ---
> > 
> >  virt/kvm/assigned-dev.c |    4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/virt/kvm/assigned-dev.c b/virt/kvm/assigned-dev.c
> > index 01f572c..e804d14 100644
> > --- a/virt/kvm/assigned-dev.c
> > +++ b/virt/kvm/assigned-dev.c
> > @@ -347,7 +347,7 @@ static int assigned_device_enable_host_msi(struct kvm *kvm,
> >  
> >  	dev->host_irq = dev->dev->irq;
> >  	if (request_threaded_irq(dev->host_irq, NULL,
> > -				 kvm_assigned_dev_thread_msi, 0,
> > +				 kvm_assigned_dev_thread_msi, IRQF_ONESHOT,
> >  				 dev->irq_name, dev)) {
> >  		pci_disable_msi(dev->dev);
> >  		return -EIO;
> > @@ -375,7 +375,7 @@ static int assigned_device_enable_host_msix(struct kvm *kvm,
> >  	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);
> > +					 IRQF_ONESHOT, dev->irq_name, dev);
> >  		if (r)
> >  			goto err;
> >  	}
> > 
> 



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