On Sun, Feb 07, 2021 at 04:56:01PM +0100, Christoph Hellwig wrote:
On Thu, Feb 04, 2021 at 09:40:23AM +0100, Christoph Hellwig wrote:
So one thing that has been on my mind for a while: I'd really like to kill the separate dma ops in Xen swiotlb. If we compare xen-swiotlb to swiotlb the main difference seems to be:
- additional reasons to bounce I/O vs the plain DMA capable
- the possibility to do a hypercall on arm/arm64
- an extra translation layer before doing the phys_to_dma and vice versa
- an special memory allocator
I wonder if inbetween a few jump labels or other no overhead enablement options and possibly better use of the dma_range_map we could kill off most of swiotlb-xen instead of maintaining all this code duplication?
So I looked at this a bit more.
For x86 with XENFEAT_auto_translated_physmap (how common is that?)
Juergen, Boris please correct me if I am wrong, but that XENFEAT_auto_translated_physmap only works for PVH guests?
pfn_to_gfn is a nop, so plain phys_to_dma/dma_to_phys do work as-is.
xen_arch_need_swiotlb always returns true for x86, and range_straddles_page_boundary should never be true for the XENFEAT_auto_translated_physmap case.
Correct. The kernel should have no clue of what the real MFNs are for PFNs.
So as far as I can tell the mapping fast path for the XENFEAT_auto_translated_physmap can be trivially reused from swiotlb.
That leaves us with the next more complicated case, x86 or fully cache coherent arm{,64} without XENFEAT_auto_translated_physmap. In that case we need to patch in a phys_to_dma/dma_to_phys that performs the MFN lookup, which could be done using alternatives or jump labels. I think if that is done right we should also be able to let that cover the foreign pages in is_xen_swiotlb_buffer/is_swiotlb_buffer, but in that worst case that would need another alternative / jump label.
For non-coherent arm{,64} we'd also need to use alternatives or jump labels to for the cache maintainance ops, but that isn't a hard problem either.