On Mon, Jan 07, 2013 at 11:39:18AM +0200, Gleb Natapov wrote: > 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. > It looks like Linux prefer to use APIC timer anyway. > > 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. -- 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
