On 16.03.23 14:53, Alex Deucher wrote:
On Thu, Mar 16, 2023 at 9:48 AM Juergen Gross <jgross@xxxxxxxx> wrote:On 16.03.23 14:45, Alex Deucher wrote:On Thu, Mar 16, 2023 at 3:50 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:On 16.03.2023 00:25, Stefano Stabellini wrote:On Wed, 15 Mar 2023, Jan Beulich wrote: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.In short, it is going to work as long as Linux has guest physical addresses (not machine addresses, those could be anything) lower than 4GB. If the address-restricted device does DMA via an IOMMU, then the device gets programmed by Linux using its guest physical addresses (not machine addresses). The 32-bit restriction would be applied by Linux to its choice of guest physical address to use to program the device, the same way it does on native. The device would be fine as it always uses Linux-provided <4GB addresses. After the IOMMU translation (pagetable setup by Xen), we could get any address, including >4GB addresses, and that is expected to work.I understand that's the "normal" way of working. But whatever the swiotlb is used for in baremetal Linux, that would similarly require its use in PVH (or HVM) aiui. So unconditionally disabling it in PVH would look to me like an incomplete attempt to disable its use altogether on x86. What difference of PVH vs baremetal am I missing here?swiotlb is not usable for GPUs even on bare metal. They often have hundreds or megs or even gigs of memory mapped on the device at any given time. Also, AMD GPUs support 44-48 bit DMA masks (depending on the chip family).But the swiotlb isn't per device, but system global.Sure, but if the swiotlb is in use, then you can't really use the GPU. So you get to pick one.
The swiotlb is used only for buffers which are not within the DMA mask of a device (see dma_direct_map_page()). So an AMD GPU supporting a 44 bit DMA mask won't use the swiotlb unless you have a buffer above guest physical address of 16TB (so basically never). Disabling swiotlb in such a guest would OTOH mean, that a device with only 32 bit DMA mask passed through to this guest couldn't work with buffers above 4GB. I don't think this is acceptable. Juergen
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