Hi, 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 ARCH_HAS_SG_CHAIN). 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 CONFIG_TRANSPARENT_HUGEPAGE is disabled? - Should riscv support ARCH_HAS_SG_CHAIN? Thanks for any help! [0] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/drivers/gpu/drm/ttm/ttm_page_alloc.c?id=fdb1a2236b07948e83e0a777e1795d4f07e52c33 [1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/drivers/gpu/drm/ttm/ttm_page_alloc.c?id=ae937fe19636067ec5e20d7f1fa10c6cc6000b52 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
