virtio-gpu without ARCH_HAS_SG_CHAIN

Michael Forney mforney at
Mon Oct 29 12:46:34 PDT 2018


I was looking at adding virtio-gpu support to tinyemu
( I got it to work on x86, but just for
fun I tried it under riscv and ran into an issue with buffer
allocations (though, this should affect any arch without

virtio-gpu uses ttm to allocate buffers, which swaps pages to ensure
that they aren't consecutive[0][1]. However, this causes
sg_alloc_table_from_pages to use a sg entry for every single page,
limiting buffers to only 170 pages (the number of sg entries that can
fit into a page). This is only 417x417 with 32bpp. I believe the page
swapping also makes TRANSFER_TO_HOST_2D inefficient by forcing the
host to do many memcpys instead of just a few.

Commenting out the swaps in [0] and [1] makes it work with larger
buffers. I don't know anything about huge pages, or how sg chaining
works, so I have a few questions:
- Is virtio-gpu intended to be usable without ARCH_HAS_SG_CHAIN?
- Is it still necessary to swap the pages when
- Should riscv support ARCH_HAS_SG_CHAIN?

Thanks for any help!


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