Re: [PATCH] KVM: arm/arm64: BUG: Fix losing level-sensitive interrupts

[Christoffer Dall <christoffer.dall@linaro.org>]

On Wed, Aug 26, 2015 at 09:27:21AM +0100, Marc Zyngier wrote:
> On Wed, 26 Aug 2015 09:46:03 +0300
> Pavel Fedin <p.fedin@samsung.com> wrote:
> 
> Hi Pavel,
> 
> > Commit 71760950bf3dc796e5e53ea3300dec724a09f593
> > ("arm/arm64: KVM: add a common vgic_queue_irq_to_lr fn") introduced
> > vgic_queue_irq_to_lr() function which checks vgic_dist_irq_is_pending()
> > before setting LR_STATE_PENDING bit. However, in some cases, the following
> > race condition is possible:
> > 1. Userland injects an IRQ with level == 1, this ends up in
> >    vgic_update_irq_pending(), which in turn calls
> >    vgic_dist_irq_set_pending() for this IRQ.
> > 2. vCPU gets kicked. But kernel does not manage to reschedule it quickly
> >    (!!!)
> > 3. Userland quickly resets the IRQ to level == 0. vgic_update_irq_pending()
> >    in this case will call vgic_dist_irq_clear_pending() and reset the
> >    pending flag.
> 
> So userspace drops the line to 0 *before* the guest had a chance to do
> anything? Well, this is not the expected behaviour for a level
> triggered interrupt, which should look like this:
> 
> - device raises the interrupt line
> - guest takes the interrupt
> - guest pokes the device to clear the interrupt condition
> - device lowers the line
> 
> The behaviour you describe is that of an edge triggered interrupt, and
> it is not surprising at all that you loose interrupts.
> 
> This really feels like a userspace bug to me (I vaguely remember some
> QEMU issues regarding this a while ago, but my memory is a bit hazy).
> Christoffer?
> 
I think it's perfectly valid for userspace to raise and lower a level
triggered interrupt at will for some device emulation.

But it is inconsistent to get to a point in the vgic code where we try
to queue something which is neither active nor pending.  See my reply to
the original patch.

-Christoffer