Re: [RFC PATCH 1/5] x86/xen: disable swiotlb for xen pvh

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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.