On Thu, Sep 03, 2015 at 04:23:04PM +0100, Marc Zyngier wrote: > On 30/08/15 14:54, Christoffer Dall wrote: > > Mapped interrupts on arm/arm64 is a tricky concept and the way we deal > > with them is not apparently easy to understand by reading various specs. > > > > Therefore, add a proper documentation file explaining the flow and > > rationale of the behavior of the vgic. > > > > Some of this text was contributed by Marc Zyngier. > > > > Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx> > > --- > > Documentation/virtual/kvm/arm/vgic-mapped-irqs.txt | 59 ++++++++++++++++++++++ > > 1 file changed, 59 insertions(+) > > create mode 100644 Documentation/virtual/kvm/arm/vgic-mapped-irqs.txt > > > > diff --git a/Documentation/virtual/kvm/arm/vgic-mapped-irqs.txt b/Documentation/virtual/kvm/arm/vgic-mapped-irqs.txt > > new file mode 100644 > > index 0000000..49e1357 > > --- /dev/null > > +++ b/Documentation/virtual/kvm/arm/vgic-mapped-irqs.txt > > @@ -0,0 +1,59 @@ > > +KVM/ARM VGIC Mapped Interrupts > > +============================== > > + > > +Setting the Physical Active State for Edge vs. Level Triggered IRQs > > +------------------------------------------------------------------- > > + > > +Mapped non-shared interrupts injected to a guest should always mark the > > +interrupt as active on the physical distributor. > > + > > +The reasoning for level-triggered interrupts: > > +For level-triggered interrupts, we have to mark the interrupt as active > > +on the physical distributor, because otherwise, as the line remains > > +asserted, the guest will never execute because the host will keep taking > > +interrupts. As soon as the guest deactivates the interrupt, the > > +physical line is sampled by the hardware again and the host takes a new > > +interrupt if the physical line is still asserted. > > + > > +The reasoning for edge-triggered interrupts: > > +For edge-triggered interrupts, if we set the HW bit in the LR we also > > +have to mark the interrupt as active on the physical distributor. If we > > +don't set the physical active bit and the interrupt hits again before > > +the guest has deactivated the interrupt, the interrupt goes to the host, > > +which cannot set the state to ACTIVE+PENDING in the LR, because that is > > +not supported when setting the HW bit in the LR. > > + > > +An alternative could be to not use HW bit at all, and inject > > +edge-triggered interrupts from a physical assigned device as pure > > +virtual interrupts, but that would potentially slow down handling of the > > +interrupt in the guest, because a physical interrupt occurring in the > > +middle of the guest ISR would preempt the guest for the host to handle > > +the interrupt. > > It would be worth mentioning that this is valid for PPIs and SPIs. LPIs > do not have an Active state (they are either Pending or not), so we'll > have to deal with edge interrupts as you just described at some point. > Other architectures do something similar, I'd expect. > > > + > > + > > +Life Cycle for Forwarded Physical Interrupts > > +-------------------------------------------- > > + > > +By forwarded physical interrupts we mean interrupts presented to a guest > > +representing a real HW event originally signaled to the host as a > > s/signaled/signalled/ > Actaully this was my first version as well, but aspell told me it was spelled signaled. Turns out it's mostly acceptable to use both spellings: http://www.merriam-webster.com/dictionary/signaled -Christoffer -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
