On Mon, 2023-11-27 at 13:51 -0400, Jason Gunthorpe wrote: > On Mon, Nov 27, 2023 at 06:43:11PM +0100, Niklas Schnelle wrote: > > > Also it turns out the writeq() loop we had so far does not produce the > > needed 64 byte TLP on s390 either so this actually makes us newly pass > > this test. > > Ooh, that is a significant problem - the userspace code won't be used > unless this test passes. So we need this on S390 to fix a bug as well > :\ > > Thanks, > Jason > Yes ;-( In the meantime I also found out that zpci_write_block(dst, src, 64) is not correct for all cases because the PCI store block requires the (pseudo-)MMIO write not to cross a 4K boundary and we need src/dst to be double word aligned. In rdma-core this is neatly handled by the get_max_write_size() but the kernel variant of that zpci_get_max_write_size() isn't just a lot harder to read and likely less efficient but also too strict thus breaking the 64 byte write up needlessly. In total we have 5 conditions for the PCI block stores: 1. The dst+len must not cross a 4K boundary in the (pseudo-)MMIO space 2. The length must not exceed the maximum write size 3. The length must be a multiple of 8 4. The src needs to be double word aligned 5. The dst needs to be double word aligned So I think a good solution would be to improve zpci_memcpy_toio() with an enhanced zpci_get_max_write_size() based on the code in rdma-core extended to also handle the alignment and length restrictions which in rdma-core are already assumed (see comment there). Then we can use zpci_memcpy_toio(dst, src, 64) for memcpy_toio_64() and rely on the compiler to optimize out the unnecessary checks (2, 3 and possibly 4, 5). So yeah this is getting a bit more complicated than originally thought. Let me cook up a patch. Thanks, Niklas
