On Thu, May 13, 2021 at 04:46:17PM +0800, Bin Meng wrote:
At present U-Boot SPL fails to boot on SiFive Unleashed board, due to a load address misaligned exception happens when loading the FIT image in spl_load_simple_fit(). The exception happens in memmove() which is called by fdt_splice_().
Commit 8f0dc4cfd106 introduces an assembly version of memmove but it does take misalignment into account (it checks if length is a multiple of machine word size but pointers need also be aligned). As a result it will generate misaligned load/store for the majority of cases and causes significant performance regression on hardware that traps misaligned load/store and emulate them using firmware.
The current behaviour of memcpy is that it checks if both src and dest pointers are co-aligned (aka congruent modular SZ_REG). If aligned, it will copy data word-by-word after first aligning pointers to word boundary. If src and dst are not co-aligned, however, byte-wise copy will be performed.
This patch was taken from the Linux kernel patch [1], which has not been applied at the time being. It fixes the memmove and optimises memcpy for misaligned cases. It will first align destination pointer to word-boundary regardless whether src and dest are co-aligned or not. If they indeed are, then wordwise copy is performed. If they are not co-aligned, then it will load two adjacent words from src and use shifts to assemble a full machine word. Some additional assembly level micro-optimisation is also performed to ensure more instructions can be compressed (e.g. prefer a0 to t6).
With this patch, U-Boot boots again on SiFive Unleashed board.
[1] https://patchwork.kernel.org/project/linux-riscv/patch/20210216225555.4976-1...
Fixes: 8f0dc4cfd106 ("riscv: assembler versions of memcpy, memmove, memset") Signed-off-by: Bin Meng bmeng.cn@gmail.com
Reviewed-by: Leo Yu-Chi Liang ycliang@cs.nctu.edu.tw