On Mon, Oct 29, 2018 at 12:46:34PM -0700, Michael Forney wrote: > 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]. Interesting. While hacking the virtio-gpu ttm code I've already noticed that I get non-contignous memory even for small allocations (cursor, which is only 4 pages), but havn't found the time yet to look at this. Christian, care to explain the background? The commit message sounds a bit like it papers over a bug somewhere else. > 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. Probably not *that* bad, the amount of data copyed doesn't change after all. But, yes, I'd prefer to have shorter sh lists too. cheers, Gerd > [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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
