Showing posts from May, 2023

lavapipe and sparse memory bindings: part two

 Thanks for all the suggestions, on here, and on twitter and on mastodon, anyway who noted I could use a single fd and avoid all the pain was correct! I hacked up an ever growing ftruncate/madvise memfd and it seemed to work fine. In order to use it for sparse I have to use it for all device memory allocations in lavapipe which means if I push forward I probably have to prove it works and scales a bit better to myself. I suspect layering some of the pb bufmgr code on top of an ever growing fd might work, or maybe just having multiple 2GB buffers might be enough. Not sure how best to do shaderResourceResidency, userfaultfd might be somewhat useful, mapping with PROT_NONE and then using write(2) to get a -EFAULT is also promising, but I'm not sure how best to avoid segfaults for read/writes to PROT_NONE regions. Once I got that going, though I ran headfirst into something that should have been obvious to me, but I hadn't thought through. llvmpipe allocates all it's textures l

lavapipe and sparse memory bindings

Mike nerdsniped me into wondering how hard sparse memory support would be in lavapipe. The answer is unfortunately extremely. Sparse binding essentially allows creating a vulkan buffer/image of a certain size, then plugging in chunks of memory to back it in page-size multiple chunks. This works great with GPU APIs where we've designed this, but it's actually hard to pull off on the CPU. Currently lavapipe allocates memory with an aligned malloc. It allocates objects with no backing and non-sparse bindings connect objects to the malloced memory. However with sparse objects, the object creation should allocate a chunk of virtual memory space, then sparse binding should bind allocated device memory into the virtual memory space. Except Linux has no interfaces for doing this without using a file descriptor. You can't mmap a chunk of anonymous memory that you allocated with malloc to another location. So if I malloc backing memory A at 0x1234000, but the virtual memory I've