[PATCH 1/2] crypto: zstd - Avoid redundant cstream initialization

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

zstd_compress() initializes the shared workspace as a CStream before
entering the walk loop. If the first source and destination fragments
each span the whole request it then hands off to zstd_compress_one(),
which initializes that same ctx->wksp as a CCtx, discarding the CStream
setup without having compressed a byte. zswap always takes this one-shot
path when storing, so every page it stores paid for both.

Neither is cheap: zstd_init_cstream() redoes the cwksp layout, zeroes the
ZSTD_CCtx, probes for BMI2 through ZSTD_cpuid(), then resets the session
and parameters and replays ten validated ZSTD_CCtx_setParameter() calls.

Defer the CStream initialization to the first walk iteration that needs
it, guarded by a flag because that iteration can be reached more than
once. The first inner iteration either takes the one-shot path and
returns or initializes the CStream, so the trailing zstd_end_stream()
cannot pick up the stale context left in ctx->cctx by an earlier request.
Unlike the old call site the new one runs with the walk's fragments
mapped, so it has to release them before failing.

For a 4 KB crypto_acomp benchmark for compression, twelve runs of nine
30K operation rounds, on the bare-metal host the median per-round mean
request time fell from 52,283 ns to 51,038 ns (2.4%). In the one-vCPU
KVM guest it fell from 16,675 ns to 15,050 ns (9.8%).
The larger improvement in guest is because of the pair of CPUID
instructions in ZSTD_cpuid() that the removed initialization runs.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 crypto/zstd.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/crypto/zstd.c b/crypto/zstd.c
index 556f5d2bdd5fb..d64472f3e11e5 100644
--- a/crypto/zstd.c
+++ b/crypto/zstd.c
@@ -96,6 +96,7 @@ static int zstd_compress_one(struct acomp_req *req, struct zstd_ctx *ctx,
 
 static int zstd_compress(struct acomp_req *req)
 {
+	bool stream_initialized = false;
 	struct crypto_acomp_stream *s;
 	unsigned int pos, scur, dcur;
 	unsigned int total_out = 0;
@@ -115,12 +116,6 @@ static int zstd_compress(struct acomp_req *req)
 	if (ret)
 		goto out;
 
-	ctx->cctx = zstd_init_cstream(&ctx->params, 0, ctx->wksp, ctx->wksp_size);
-	if (!ctx->cctx) {
-		ret = -EINVAL;
-		goto out;
-	}
-
 	do {
 		dcur = acomp_walk_next_dst(&walk);
 		if (!dcur) {
@@ -142,6 +137,19 @@ static int zstd_compress(struct acomp_req *req)
 				goto out;
 			}
 
+			if (!stream_initialized) {
+				ctx->cctx = zstd_init_cstream(&ctx->params, 0,
+							      ctx->wksp, ctx->wksp_size);
+				if (!ctx->cctx) {
+					/* Release in the reverse of the map order. */
+					acomp_walk_done_src(&walk, 0);
+					acomp_walk_done_dst(&walk, 0);
+					ret = -EINVAL;
+					goto out;
+				}
+				stream_initialized = true;
+			}
+
 			if (scur) {
 				inbuf.pos = 0;
 				inbuf.src = walk.src.virt.addr;
-- 
2.53.0-Meta