Messages in this thread | | | Date | Thu, 16 Mar 2023 14:48:42 +0100 | Subject | Re: [RFC PATCH 1/5] x86/xen: disable swiotlb for xen pvh | From | Juergen Gross <> |
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On 16.03.23 14:45, Alex Deucher wrote: > On Thu, Mar 16, 2023 at 3:50 AM Jan Beulich <jbeulich@suse.com> 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.
Juergen
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