svcrdma: Clean up RPC-over-RDMA Reply header encoder

Replace C structure-based XDR decoding with pointer arithmetic.
Pointer arithmetic is considered more portable, and is used
throughout the kernel's existing XDR encoders. The gcc optimizer
generates similar assembler code either way.

Byte-swapping before a memory store on x86 typically results in an
instruction pipeline stall. Avoid byte-swapping when encoding a new
header.

svcrdma currently doesn't alter a connection's credit grant value
after the connection has been accepted, so it is effectively a
constant. Cache the byte-swapped value in a separate field.

Christoph suggested pulling the header encoding logic into the only
function that uses it.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

3 files changed, 14 insertions, 24 deletions

diff --git a/net/sunrpc/xprtrdma/svc_rdma_marshal.c b/net/sunrpc/xprtrdma/svc_rdma_marshal.c
index 4e72034..c7d15b1 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_marshal.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_marshal.c
@@ -249,7 +249,7 @@ int svc_rdma_xdr_encode_error(struct svcxprt_rdma *xprt,
 
 	*va++ = rmsgp->rm_xid;
 	*va++ = rmsgp->rm_vers;
-	*va++ = cpu_to_be32(xprt->sc_max_requests);
+	*va++ = xprt->sc_fc_credits;
 	*va++ = rdma_error;
 	*va++ = cpu_to_be32(err);
 	if (err == ERR_VERS) {
@@ -329,19 +329,3 @@ void svc_rdma_xdr_encode_array_chunk(struct rpcrdma_write_array *ary,
 	seg->rs_offset = rs_offset;
 	seg->rs_length = cpu_to_be32(write_len);
 }
-
-void svc_rdma_xdr_encode_reply_header(struct svcxprt_rdma *xprt,
-				  struct rpcrdma_msg *rdma_argp,
-				  struct rpcrdma_msg *rdma_resp,
-				  enum rpcrdma_proc rdma_type)
-{
-	rdma_resp->rm_xid = rdma_argp->rm_xid;
-	rdma_resp->rm_vers = rdma_argp->rm_vers;
-	rdma_resp->rm_credit = cpu_to_be32(xprt->sc_max_requests);
-	rdma_resp->rm_type = cpu_to_be32(rdma_type);
-
-	/* Encode <nul> chunks lists */
-	rdma_resp->rm_body.rm_chunks[0] = xdr_zero;
-	rdma_resp->rm_body.rm_chunks[1] = xdr_zero;
-	rdma_resp->rm_body.rm_chunks[2] = xdr_zero;
-}
diff --git a/net/sunrpc/xprtrdma/svc_rdma_sendto.c b/net/sunrpc/xprtrdma/svc_rdma_sendto.c
index ba76f1617..515221b 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_sendto.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_sendto.c
@@ -560,12 +560,12 @@ int svc_rdma_sendto(struct svc_rqst *rqstp)
 	struct rpcrdma_msg *rdma_argp;
 	struct rpcrdma_msg *rdma_resp;
 	struct rpcrdma_write_array *wr_ary, *rp_ary;
-	enum rpcrdma_proc reply_type;
 	int ret;
 	int inline_bytes;
 	struct page *res_page;
 	struct svc_rdma_req_map *vec;
 	u32 inv_rkey;
+	__be32 *p;
 
 	dprintk("svcrdma: sending response for rqstp=%p\n", rqstp);
 
@@ -597,12 +597,17 @@ int svc_rdma_sendto(struct svc_rqst *rqstp)
 	if (!res_page)
 		goto err0;
 	rdma_resp = page_address(res_page);
-	if (rp_ary)
-		reply_type = RDMA_NOMSG;
-	else
-		reply_type = RDMA_MSG;
-	svc_rdma_xdr_encode_reply_header(rdma, rdma_argp,
-					 rdma_resp, reply_type);
+
+	p = &rdma_resp->rm_xid;
+	*p++ = rdma_argp->rm_xid;
+	*p++ = rdma_argp->rm_vers;
+	*p++ = rdma->sc_fc_credits;
+	*p++ = rp_ary ? rdma_nomsg : rdma_msg;
+
+	/* Start with empty chunks */
+	*p++ = xdr_zero;
+	*p++ = xdr_zero;
+	*p   = xdr_zero;
 
 	/* Send any write-chunk data and build resp write-list */
 	if (wr_ary) {
diff --git a/net/sunrpc/xprtrdma/svc_rdma_transport.c b/net/sunrpc/xprtrdma/svc_rdma_transport.c
index ca2799a..174928f 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_transport.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_transport.c
@@ -1002,6 +1002,7 @@ static struct svc_xprt *svc_rdma_accept(struct svc_xprt *xprt)
 	newxprt->sc_max_req_size = svcrdma_max_req_size;
 	newxprt->sc_max_requests = min_t(u32, dev->attrs.max_qp_wr,
 					 svcrdma_max_requests);
+	newxprt->sc_fc_credits = cpu_to_be32(newxprt->sc_max_requests);
 	newxprt->sc_max_bc_requests = min_t(u32, dev->attrs.max_qp_wr,
 					    svcrdma_max_bc_requests);
 	newxprt->sc_rq_depth = newxprt->sc_max_requests +