virtio-gpu without ARCH_HAS_SG_CHAIN
mforney at mforney.org
Mon Oct 29 12:46:34 PDT 2018
I was looking at adding virtio-gpu support to tinyemu
(https://bellard.org/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. 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  and  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
CONFIG_TRANSPARENT_HUGEPAGE is disabled?
- Should riscv support ARCH_HAS_SG_CHAIN?
Thanks for any help!
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