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:

quoted

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.