On 2016-05-03 04:03, Nadav Amit wrote: > Yang Zhang <yang.zhang.wz@xxxxxxxxx> wrote: > >> I think it is not only interrupt. There must have the DMAR emulation and >> the cost for DMA is heavy in VM(DMA operations are very frequently). I >> cannot remember whether there are strong dependency in hardware between >> DMAR and IR(I know IR is relying on QI). Even hardware dependency is ok, >> is it ok for OS running in hardware with IR but without DMAR? > > Do you know a way for the IOMMU to report that DMAR is disabled, while IR > is enabled? The hardware cannot decide about disabling this, but the guest can, of course. In fact, you can even configure Linux to have DMAR off by default until you pass "intel_iommu=on" on the command line (I think distros still do this - at least they used to). No idea about other OSes, though. > > Anyhow, the VM can use IOMMU passthrough mode to avoid most IOMMU overhead. > Regardless, a recent patch-set should improve DMAR performance > considerably [1]. The bottleneck with emulated DMAR is rather in QEMU. But DMAR can be almost as cheap as IR once we get it running for VFIO and vhost: both need proper caching because they do not work with QEMU in the loop for each and every DMA transfer. Still no need to deviate from physical hardware. Jan -- Siemens AG, Corporate Technology, CT RDA ITP SES-DE Corporate Competence Center Embedded Linux -- 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
