net/smc: Decouple sf and attached send_buf in smc_loopback

Before this patch there was the following assumption in
smc_loopback.c>smc_lo_move_data():
sf (signalling flag) == 0 : data is already in an attached target dmb
sf == 1 : data is not yet in the target dmb

This is true for the 2 callers in smc client
smcd_cdc_msg_send() : sf=1
smcd_tx_rdma_writes() : sf=0
but should not be a general assumption.

Add a bool to struct smc_buf_desc to indicate whether an SMC-D sndbuf_desc
is an attached buffer. Don't call move_data() for attached send_buffers,
because it is not necessary.

Move the data in smc_lo_move_data() if len != 0 and signal when requested.

Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Mahanta Jambigi <mjambigi@linux.ibm.com>
Reviewed-by: Dust Li <dust.li@linux.alibaba.com>
Link: https://patch.msgid.link/20250918110500.1731261-3-wintera@linux.ibm.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/smc/smc_core.h

@@ -13,6 +13,7 @@
 #define _SMC_CORE_H
 
 #include <linux/atomic.h>
+#include <linux/types.h>
 #include <linux/smc.h>
 #include <linux/pci.h>
 #include <rdma/ib_verbs.h>
@@ -221,6 +222,10 @@ struct smc_buf_desc {
 					/* virtually contiguous */
 		};
 		struct { /* SMC-D */
+			/* SMC-D tx buffer */
+			bool		is_attached;
+					/* no need for explicit writes */
+			 /* SMC-D rx buffer: */
 			unsigned short	sba_idx;
 					/* SBA index number */
 			u64		token;

net/smc/smc_ism.c

@@ -274,6 +274,7 @@ int smc_ism_attach_dmb(struct smcd_dev *dev, u64 token,
 		dmb_desc->cpu_addr = dmb.cpu_addr;
 		dmb_desc->dma_addr = dmb.dma_addr;
 		dmb_desc->len = dmb.dmb_len;
+		dmb_desc->is_attached = true;
 	}
 	return rc;
 }

net/smc/smc_loopback.c

@@ -224,12 +224,6 @@ static int smc_lo_move_data(struct smcd_dev *smcd, u64 dmb_tok,
 	struct smc_lo_dev *ldev = smcd->priv;
 	struct smc_connection *conn;
 
-	if (!sf)
-		/* since sndbuf is merged with peer DMB, there is
-		 * no need to copy data from sndbuf to peer DMB.
-		 */
-		return 0;
-
 	read_lock_bh(&ldev->dmb_ht_lock);
 	hash_for_each_possible(ldev->dmb_ht, tmp_node, list, dmb_tok) {
 		if (tmp_node->token == dmb_tok) {
@@ -244,6 +238,9 @@ static int smc_lo_move_data(struct smcd_dev *smcd, u64 dmb_tok,
 	memcpy((char *)rmb_node->cpu_addr + offset, data, size);
 	read_unlock_bh(&ldev->dmb_ht_lock);
 
+	if (!sf)
+		return 0;
+
 	conn = smcd->conn[rmb_node->sba_idx];
 	if (!conn || conn->killed)
 		return -EPIPE;

net/smc/smc_tx.c

@@ -426,6 +426,9 @@ static int smcd_tx_rdma_writes(struct smc_connection *conn, size_t len,
 	int srcchunk, dstchunk;
 	int rc;
 
+	if (conn->sndbuf_desc->is_attached)
+		return 0;
+
 	for (dstchunk = 0; dstchunk < 2; dstchunk++) {
 		for (srcchunk = 0; srcchunk < 2; srcchunk++) {
 			void *data = conn->sndbuf_desc->cpu_addr + src_off;