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_q...@miniinfo.net>
> ---
> 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.
