On Thu, 2020-11-05 at 16:09 +, David Woodhouse wrote:
> From: David Woodhouse
>
> This ended up with an odd mix of recursion (albeit *mostly*
> tail-recursion) and interation that could have been prettier. In
> addition, while recursing it potentially adjusted op->count which is
> used by callers to see the amount of I/O actually performed.
>
> Fix it by bringing nvme_build_prpl() into the normal loop using 'i'
> as the offset in the op.
>
> Fixes: 94f0510dc ("nvme: Split requests by maximum allowed size")
> Signed-off-by: David Woodhouse
> ---
> v2: Fix my bug with checking a u16 for being < 0.
> Fix the bug I inherited from commit 94f0510dc but made unconditional.
>
> src/hw/nvme.c | 77 ++-
> 1 file changed, 33 insertions(+), 44 deletions(-)
>
Now, on top of that we *could* do something like this...
--- a/src/hw/nvme.c
+++ b/src/hw/nvme.c
@@ -742,6 +742,15 @@ nvme_cmd_readwrite(struct nvme_namespace *ns, struct
disk_op_s *op, int write)
blocks = blocks_remaining < max_blocks ? blocks_remaining
: max_blocks;
+if (blocks < blocks_remaining && ns->block_size < NVME_PAGE_SIZE &&
+!(((u32)op_buf) & (ns->block_size-1))) {
+u32 align = ((u32)op_buf & (NVME_PAGE_SIZE - 1)) /
ns->block_size;
+if (blocks > (max_blocks - align)) {
+dprintf(3, "Restrain to %u blocks to align (buf %p
size %u/%u)\n", max_blocks - align, op_buf, NVME_PAGE_SIZE, ns->block_size);
+blocks = max_blocks - align;
+}
+}
+
if (write) {
memcpy(ns->dma_buffer, op_buf, blocks * ns->block_size);
}
While debugging this I watched a boot sector, after being loaded at
:7c00, load another 63 sectors at :7e00. It didn't get to use
the prpl at all, and looked something like this...
ns 1 read lba 0+1: 0
Booting from :7c00
ns 1 read lba 1+8: 0
ns 1 read lba 9+8: 0
ns 1 read lba 17+8: 0
ns 1 read lba 25+8: 0
ns 1 read lba 33+8: 0
ns 1 read lba 41+8: 0
ns 1 read lba 49+8: 0
ns 1 read lba 57+7: 0
If we make an *attempt* to align it, such as the proof-of-concept shown
above, then it ends up getting back in sync:
ns 1 read lba 0+1: 0
Booting from :7c00
Restrain to 1 blocks to align (buf 0x7e00 size 4096/512)
ns 1 read lba 1+1: 0
ns 1 read lba 1+62: 0
I just don't know that I care enough about the optimisation to make it
worth the ugliness of that special case in the loop, which includes a
division.
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