Date:	Thu, 16 Sep 2010 14:57:17 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] kvm: enable irq injection from interrupt context

On Thu, Sep 16, 2010 at 02:33:01PM +0200, Gleb Natapov wrote:
> On Thu, Sep 16, 2010 at 02:13:38PM +0200, Michael S. Tsirkin wrote:
> > > > We haver two users: qemu does deasserts, vhost-net does asserts.
> > > Well this is broken. You want KVM to track level for you and this is
> > > wrong. KVM does this anyway because it can't relay on devise model
> > > to behave correctly [0], but in your case it is designed to behave
> > > incorrectly.
> > > 
> > > Interrupt type is a device property. PCI devices just happen to be level
> > > triggered according to PCI spec. What if you want to use vhost-net to
> > > implement network device which has active-low interrupt line? [1]
> > 
> > The polarity would have to be reversed in gsi (irq line can be shared,
> > all devices must be active high or low consistently).
> > 
> There are gsi dedicated to PCI. They can be shared only between PCI
> devices.
> 
> > > If you want to split parts that asserts irq and de-asserts it then we
> > > should have irqfd that tracks line status and knows interrupt line
> > > polarity.
> > 
> > Yes, it can know about polarity even though I think it's cleaner to do this
> > per gsi. But it can not track line status as line is shared with
> > other devices.
> It should track only device's line status.

There is no such thing as device's line status on real hardware, either.
Devices do not drive INT# high: they drive it low (all the time)
or do not drive it at all.

Or consider express, the spec explicitly says:
"Note: Duplicate Assert_INTx/Deassert_INTx Messages have no effect, but
 are not errors."

> > 
> > > > Another application is out of process virtio (sandboxing!).
> > > It will still assert and de-assert irq at the same code, so it will be
> > > able to track irq line status.
> > > 
> > > > Again, pci stuff needs to stay in qemu.
> > > > 
> > > 
> > > Nothing to do with PCI whatsoever.
> > > 
> > > [0] most qemu devices behave incorrectly and trigger level irq more then
> > >     needed.
> > 
> > Which devices?
> Most of them. They just call update_irq_status() or something and
> re-assert interrupt regardless of what previous status was.

At least for PCI devices, these calls do nothing if status does not change.

> > pci core tracks line status and will never assert the same
> > line multiple times.
> That's good if pci core does this, but device shouldn't even try it.

I disagree. We don't want to duplicate a ton of code all over
the codebase.

> > 
> > > [1] this is how correct PCI device should behave but we override
> > >     polarity in ACPI, but now incorrect behaviour is deeply designed
> > >     into vhost-net.
> > 
> > Not really, vhost net signals an eventfd. What happens then is
> > up to kvm.
> > 
> That is what current broken design does and it works, but if you want to
> save unneeded calls into kvm fix design.

The interface seems clean enough: vhost handles virtio ring, qemu/kvm handle pci.
Making vhost aware of pci breaks this, I would not call that fixing the
design.

> --
> 			Gleb.
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