Re: [Qemu-devel] [PATCH KVM v2 1/4] KVM: fix i8254 IRQ0 to be normally high

[Gleb Natapov]

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 <address@hidden>
> > ---
> >  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.