Gentlepeople, We're currently working on PCI device pass-through support for KVM. In this model all physical hardware access to specific devices will be performed by the VM, and not by the host.
In particular, this requires interrupt handling to be done by the guest -- The host shouldn't load the corresponding device driver or otherwise access the device. Since the host kernel is not aware of the device semantics it cannot acknowledge the interrupt at the device level. As far as the host kernel is concerned the VM is a user level process. We require the ability to forward interrupt handling to such entities. The current kernel interrupt handling path doesn't allow deferring interrupt handling _and_ acknowledgement. We believe that this requires changing the current interrupt path in several fashions: 0. Adding an IRQ_DEFERRED mechanism to the interrupt handling path. ISRs returning IRQ_DEFERRED will keep the interrupt masked until future acknowledge. 1. Deferred acknowledge mechanism which would acknowledge the APIC and unmask the interrupt. An alternative idea was suggested at http://lists.xensource.com/archives/html/xen-devel/2007-05/msg01148.html --- which is based on reversing the polarity of the received interrupt. Once the guest acknowledges the interrupt at the device level, the polarity is reversed again by the host. This may not be the best choice performance wise -- twice the interrupt count. Another issue we are aware of is the effect deferred handling will have on other devices sharing the interrupt. This seems to be a problem regardless of the solution. We wouldn't like to assume VT-d availability right now. Any ideas ? Thoughts ? -- Ors - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
