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 ? Thanks, Pingfan > performance. The hypervisor being able to direct interrupts to a target > CPU also allows it the ability to filter interrupts and prevent the > device from signaling spurious or malicious interrupts. This is > particularly important with message signaled interrupts since any device > capable of DMA is able to inject random MSIs into the host. The > compatibility side is a feature of Intel platforms supporting x2apic. > The interrupt remapper provides a translation layer to allow xapic aware > hardware, such as ioapics, to function when the processors are switched > to x2apic mode. 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 -- 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
