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. Thanks, Alex -- 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
