On Wed, Dec 26, 2012 at 10:39:53PM -0700, Matthew Ogilvie wrote: > Reading the spec, it is clear that most modes normally leave the IRQ > output line high, and only pulse it low to generate a leading edge. > Especially the most commonly used mode 2. > > The KVM i8254 model does not try to emulate the duration of the pulse at > all, so just swap the high/low settings it to leave it high most of > the time. > > This fix is a prerequisite to improving the i8259 model to handle > the trailing edge of an interupt request as indicated in its spec: > If it gets a trailing edge of an IRQ line before it starts to service > the interrupt, the request should be canceled. > > See http://bochs.sourceforge.net/techspec/intel-82c54-timer.pdf.gz > or search the net for 23124406.pdf. > > Risks: > > There is a risk that migrating a running guest between versions > with and without this patch will lose or gain a single timer > interrupt during the migration process. The only case where Can you elaborate on how exactly this can happen? Do not see it. > this is likely to be serious is probably losing a single-shot (mode 4) > interrupt, but if my understanding of how things work is good, then > that should only be possible if a whole slew of conditions are > all met: > > 1. The guest is configured to run in a "tickless" mode (like > modern Linux). > 2. The guest is for some reason still using the i8254 rather > than something more modern like an HPET. (The combination > of 1 and 2 should be rare.) This is not so rare. For performance reason it is better to not have HPET at all. In fact -no-hpet is how I would advice anyone to run qemu. > 3. The migration is going from a fixed version back to the > old version. (Not sure how common this is, but it should > be rarer than migrating from old to new.) > 4. There are not going to be any "timely" events/interrupts > (keyboard, network, process sleeps, etc) that cause the guest > to reset the PIT mode 4 one-shot counter "soon enough". > > This combination should be rare enough that more complicated > solutions are not worth the effort. > > Signed-off-by: Matthew Ogilvie <mmogilvi_qemu@xxxxxxxxxxxx> > --- > arch/x86/kvm/i8254.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c > index c1d30b2..cd4ec60 100644 > --- a/arch/x86/kvm/i8254.c > +++ b/arch/x86/kvm/i8254.c > @@ -290,8 +290,12 @@ static void pit_do_work(struct kthread_work *work) > } > spin_unlock(&ps->inject_lock); > if (inject) { > - kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 1); > + /* Clear previous interrupt, then create a rising > + * edge to request another interupt, and leave it at > + * level=1 until time to inject another one. > + */ > kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 0); > + kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 1); > > /* > * Provides NMI watchdog support via Virtual Wire mode. > -- > 1.7.10.2.484.gcd07cc5 -- Gleb. -- 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
