On 15.03.2023 01:52, Stefano Stabellini wrote: > On Mon, 13 Mar 2023, Jan Beulich wrote: >> On 12.03.2023 13:01, Huang Rui wrote: >>> Xen PVH is the paravirtualized mode and takes advantage of hardware >>> virtualization support when possible. It will using the hardware IOMMU >>> support instead of xen-swiotlb, so disable swiotlb if current domain is >>> Xen PVH. >> >> But the kernel has no way (yet) to drive the IOMMU, so how can it get >> away without resorting to swiotlb in certain cases (like I/O to an >> address-restricted device)? > > I think Ray meant that, thanks to the IOMMU setup by Xen, there is no > need for swiotlb-xen in Dom0. Address translations are done by the IOMMU > so we can use guest physical addresses instead of machine addresses for > DMA. This is a similar case to Dom0 on ARM when the IOMMU is available > (see include/xen/arm/swiotlb-xen.h:xen_swiotlb_detect, the corresponding > case is XENFEAT_not_direct_mapped). But how does Xen using an IOMMU help with, as said, address-restricted devices? They may still need e.g. a 32-bit address to be programmed in, and if the kernel has memory beyond the 4G boundary not all I/O buffers may fulfill this requirement. Jan
