[PATCH 0/2] crypto: zstd - avoid initializing the workspace twice

[Usama Arif <usama.arif@linux.dev>]

Both zstd_compress() and zstd_decompress() set up the shared per-CPU
workspace as a C/DStream before walking the request, and then, when the
first source and destination fragments each span the whole request, hand
off to zstd_compress_one()/zstd_decompress_one(), which immediately
overwrite that same ctx->wksp with a CCtx/DCtx. The stream setup is
discarded without a byte having been processed.

That one-shot path is not a corner case: zswap always takes it when
storing, and takes it for a load whenever the stored object lies within a
single zsmalloc page.

These two patches defer the stream initialization to the first walk
iteration that actually streams, guarded by a flag because that iteration
can be reached more than once.

A 4 KiB crypto_acomp benchmark [1], twelve runs of nine 30,000-operation
rounds. Bare metal is an Intel Xeon Platinum 8321HC, turbo off,
performance governor, pinned to one core; the VM is a one-vCPU KVM guest
on a faster host.

                  baseline    patched     delta
  bare metal
    compress      52,283 ns   51,038 ns   1,245 ns   2.4%
    decompress     2,317 ns    1,998 ns     319 ns  13.8%
  one-vCPU KVM
    compress      16,675 ns   15,050 ns   1,625 ns   9.8%
    decompress     3,516 ns    2,265 ns   1,251 ns  35.6%

The guest numbers are larger because the two CPUID instructions in
ZSTD_cpuid() become unconditional VM exits there. 

[1] https://gist.github.com/uarif1/5cf02f0e22c23f0d1b3d84348f12914c
 
Usama Arif (2):
  crypto: zstd - Avoid redundant cstream initialization
  crypto: zstd - Avoid redundant dstream initialization

 crypto/zstd.c | 40 ++++++++++++++++++++++++++++------------
 1 file changed, 28 insertions(+), 12 deletions(-)


base-commit: 4b18edbd8e70f7e6860d56370f13244896d0f95c
-- 
2.53.0-Meta