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. -- 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
