On Thu, Sep 16, 2010 at 01:17:52PM +0200, Gleb Natapov wrote: > On Thu, Sep 16, 2010 at 12:53:52PM +0200, Michael S. Tsirkin wrote: > > On Thu, Sep 16, 2010 at 12:54:03PM +0200, Gleb Natapov wrote: > > > On Thu, Sep 16, 2010 at 12:44:55PM +0200, Michael S. Tsirkin wrote: > > > > On Thu, Sep 16, 2010 at 12:20:47PM +0200, Gleb Natapov wrote: > > > > > On Thu, Sep 16, 2010 at 12:13:39PM +0200, Michael S. Tsirkin wrote: > > > > > > On Thu, Sep 16, 2010 at 12:13:32PM +0200, Gleb Natapov wrote: > > > > > > > On Thu, Sep 16, 2010 at 11:53:10AM +0200, Michael S. Tsirkin wrote: > > > > > > > > On Thu, Sep 16, 2010 at 11:46:03AM +0200, Avi Kivity wrote: > > > > > > > > > On 09/16/2010 11:25 AM, Gleb Natapov wrote: > > > > > > > > > >> > > > > > > > > > >> MSI only appeared in rhel6, older guests still use level interrupts. > > > > > > > > > >So they are already slow for other reasons. > > > > > > > > > > > > > > > > > > > > > > > > > > > > Exactly, for example they need to exit to userspace to ack the > > > > > > > > > interrupt. That's far slower than the workqueue. > > > > > > > > > > > > > > > > Well, this is not exactly comparable: you might get > > > > > > > > same irq asserted multiple times and only deasserted once. > > > > > > > > > > > > > > > Are we talking about level interrupts? Why would you assert level > > > > > > > triggered interrupt multiple times before deasserting it? > > > > > > > > > > > > User of irqfd has no way to know what current interrupt level is. > > > > > > So it has to keep asserting. > > > > > > > > > > > Why can't it keep track of current level? > > > > > > > > This breaks the model: eventfd user is unaware of PCI, levels and such: > > > > it just signals the event. Remember that asserts are done from e.g. vhost-net, > > > > deasserts need to be handled by qemu. > > > > > > > eventfd user implements HW and it knows exactly what type of interrupt > > > this HW generates. > > > > 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). > 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. > > 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? pci core tracks line status and will never assert the same line multiple times. > [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. > -- > 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
