On Mon, Oct 24, 2011 at 02:06:08PM +0200, Jan Kiszka wrote: > On 2011-10-24 13:09, Avi Kivity wrote: > > On 10/24/2011 12:19 PM, Jan Kiszka wrote: > >>> > >>> With the new feature it may be worthwhile, but I'd like to see the whole > >>> thing, with numbers attached. > >> > >> It's not a performance issue, it's a resource limitation issue: With the > >> new API we can stop worrying about user space device models consuming > >> limited IRQ routes of the KVM subsystem. > >> > > > > Only if those devices are in the same process (or have access to the > > vmfd). Interrupt routing together with irqfd allows you to disaggregate > > the device model. Instead of providing a competing implementation with > > new limitations, we need to remove the limitations of the old > > implementation. > > That depends on where we do the cut. Currently we let the IRQ source > signal an abstract edge on a pre-allocated pseudo IRQ line. But we > cannot build correct MSI-X on top of the current irqfd model as we lack > the level information (for PBA emulation). *) I don't agree here. IMO PBA emulation would need to clear pending bits on interrupt status register read. So clearing pending bits could be done by ioctl from qemu while setting them would be done from irqfd. > So we either need to > extend the existing model anyway -- or push per-vector masking back to > the IRQ source. In the latter case, it would be a very good chance to > give up on limited pseudo GSIs with static routes and do MSI messaging > from external IRQ sources to KVM directly. > But all those considerations affect different APIs than what I'm > proposing here. We will always need a way to inject MSIs in the context > of the VM as there will always be scenarios where devices are better run > in that very same context, for performance or simplicity or whatever > reasons. E.g., I could imagine that one would like to execute an > emulated IRQ remapper rather in the hypervisor context than > "over-microkernelized" in a separate process. > > Jan > > *) Realized this while trying to generalize the proposed MSI-X MMIO > acceleration for assigned devices to arbitrary device models, vhost-net, I'm actually working on a qemu patch to get pba emulation working correctly. I think it's doable with existing irqfd. > and specifically vfio. Interesting. How would you clear the pseudo interrupt level? > -- > Siemens AG, Corporate Technology, CT T DE IT 1 > Corporate Competence Center Embedded Linux -- 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
