crypto: x86/chacha - yield the FPU occasionally

To improve responsiveness, yield the FPU (temporarily re-enabling
preemption) every 4 KiB encrypted/decrypted, rather than keeping
preemption disabled during the entire encryption/decryption operation.

Alternatively we could do this for every skcipher_walk step, but steps
may be small in some cases, and yielding the FPU is expensive on x86.
Suggested-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

arch/x86/crypto/chacha_glue.c

@@ -132,6 +132,7 @@ static int chacha_simd_stream_xor(struct skcipher_request *req,
 {
 	u32 *state, state_buf[16 + 2] __aligned(8);
 	struct skcipher_walk walk;
+	int next_yield = 4096; /* bytes until next FPU yield */
 	int err;
 
 	BUILD_BUG_ON(CHACHA_STATE_ALIGN != 16);
@@ -144,12 +145,21 @@ static int chacha_simd_stream_xor(struct skcipher_request *req,
 	while (walk.nbytes > 0) {
 		unsigned int nbytes = walk.nbytes;
 
-		if (nbytes < walk.total)
+		if (nbytes < walk.total) {
 			nbytes = round_down(nbytes, walk.stride);
+			next_yield -= nbytes;
+		}
 
 		chacha_dosimd(state, walk.dst.virt.addr, walk.src.virt.addr,
 			      nbytes, ctx->nrounds);
 
+		if (next_yield <= 0) {
+			/* temporarily allow preemption */
+			kernel_fpu_end();
+			kernel_fpu_begin();
+			next_yield = 4096;
+		}
+
 		err = skcipher_walk_done(&walk, walk.nbytes - nbytes);
 	}