[RFC PATCH 6/7] crypto: x86/ghash - limit FPU preemption

[Robert Elliott]

As done by the ECB and CBC helpers in arch/x86/crypt/ecb_cbc_helpers.h,
limit the number of bytes processed between kernel_fpu_begin() and
kernel_fpu_end() calls.

Those functions call preempt_disable() and preempt_enable(), so
the CPU core is unavailable for scheduling while running, leading to:
    rcu: INFO: rcu_preempt detected expedited stalls on CPUs/tasks: {12-... } 22 jiffies s: 277 root: 0x1/.

Fixes: 0e1227d356e9 ("crypto: ghash - Add PCLMULQDQ accelerated implementation")
Suggested-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Robert Elliott <elliott@xxxxxxx>
---
 arch/x86/crypto/ghash-clmulni-intel_glue.c | 27 ++++++++++++++++------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/arch/x86/crypto/ghash-clmulni-intel_glue.c b/arch/x86/crypto/ghash-clmulni-intel_glue.c
index b25730c5b267..b9a3e2187f5b 100644
--- a/arch/x86/crypto/ghash-clmulni-intel_glue.c
+++ b/arch/x86/crypto/ghash-clmulni-intel_glue.c
@@ -25,6 +25,8 @@
 #define GHASH_BLOCK_SIZE	16
 #define GHASH_DIGEST_SIZE	16
 
+#define FPU_BYTES 4096U /* avoid kernel_fpu_begin/end scheduler/rcu stalls */
+
 void clmul_ghash_mul(char *dst, const u128 *shash);
 
 void clmul_ghash_update(char *dst, const char *src, unsigned int srclen,
@@ -81,10 +83,11 @@ static int ghash_update(struct shash_desc *desc,
 	struct ghash_desc_ctx *dctx = shash_desc_ctx(desc);
 	struct ghash_ctx *ctx = crypto_shash_ctx(desc->tfm);
 	u8 *dst = dctx->buffer;
+	unsigned int fpulen;
 
 	if (dctx->bytes) {
 		int n = min(srclen, dctx->bytes);
-		u8 *pos = dst + (GHASH_BLOCK_SIZE - dctx->bytes);
+		u8 *pos = dst + GHASH_BLOCK_SIZE - dctx->bytes;
 
 		dctx->bytes -= n;
 		srclen -= n;
@@ -99,13 +102,23 @@ static int ghash_update(struct shash_desc *desc,
 		}
 	}
 
-	kernel_fpu_begin();
-	clmul_ghash_update(dst, src, srclen, &ctx->shash);
-	kernel_fpu_end();
+	while (srclen >= GHASH_BLOCK_SIZE) {
+		fpulen = min(srclen, FPU_BYTES);
+
+		kernel_fpu_begin();
+		while (fpulen >= GHASH_BLOCK_SIZE) {
+			int n = min_t(unsigned int, fpulen, GHASH_BLOCK_SIZE);
+
+			clmul_ghash_update(dst, src, n, &ctx->shash);
+
+			srclen -= n;
+			fpulen -= n;
+			src += n;
+		}
+		kernel_fpu_end();
+	}
 
-	if (srclen & 0xf) {
-		src += srclen - (srclen & 0xf);
-		srclen &= 0xf;
+	if (srclen) {
 		dctx->bytes = GHASH_BLOCK_SIZE - srclen;
 		while (srclen--)
 			*dst++ ^= *src++;
-- 
2.37.3