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

[Stefano Stabellini]

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).

Jurgen, what do you think? Would you rather make xen_swiotlb_detect
common between ARM and x86?