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

[Gleb Natapov <gleb@redhat.com>]

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@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.