On Wed, Aug 14, 2013 at 10:50 AM, Alex Williamson <alex.williamson@xxxxxxxxxx> wrote: > On Wed, 2013-08-14 at 10:37 +0800, Liu ping fan wrote: >> On Wed, Nov 2, 2011 at 11:31 PM, Alex Williamson >> <alex.williamson@xxxxxxxxxx> wrote: >> > On Wed, 2011-11-02 at 13:26 +0800, Kai Huang wrote: >> >> Hi, >> >> >> >> In case of direct io, without the interrupt remapping in IOMMU (intel >> >> VT-d or AMD IOMMU), hypervisor needs to inject interrupt for guest >> >> when the guest is scheduled to specific CPU. At the beginning I >> >> thought with IOMMU's interrupt remapping, the hardware can directly >> >> forward the interrupt to guest without trapping into hypervisor when >> >> the interrupt happens, but after reading the Intel VT-d's manual, I >> >> found the interrupt mapping feature just add another mapping which >> >> allows software to control (mainly) the destination and vector, and we >> >> still need hypervisor to inject the interrupt when the guest is >> >> scheduled as only after the guest is scheduled, the target CPU can be >> >> known. If my understanding is correct, seems the interrupt remapping >> >> does not bring any performance improvement. So what's the benefit of >> >> IOMMU's interrupt remapping? Can someone explain the usage model of >> >> interrupt remapping in IOMMU? >> > >> > Interrupt remapping provides isolation and compatibility, not >> >> The guest can not directly program the msi-x on pci device, so msix is >> still under the control of host. Why do we need extra control >> introduced by iommu ? > > An MSI interrupt is just a DMA write with a specific address and > payload. Any device capable of DMA can theoretically inject an MSI > interrupt using other means besides the MSI/MSI-X configuration areas. Thank you for the clear explanation. Thanks and regards, Pingfan -- 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
