On Thu, Sep 16, 2010 at 02:57:17PM +0200, Michael S. Tsirkin wrote: > 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. Same thing, other naming. Device either drive it low (irq_set(1)) or not (irq_set(0)). > > 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. I am not sure what code are you locking at. e1000 device emulation doesn't check previous line status before calling qemu_set_irq(). > > > > 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. > So abstract it into a function. It shouldn't be part of PCI emulation. > > > > > > > [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. > Once again. Nothing to do with PCI, everything to do with device emulation. And I propose to abstract interrupt assertion part into irqfd, not into vhost. -- 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
