[PATCH 1/3] net/gve: fix segment TSO segment offset calculation

[Joshua Washington <joshwash@google.com>]

The descriptor limit validation for TSO packets was erroneously
computing the TSO segment offset at the end of an mbuf. The first bytes
of any mbuf will be included as part of the previous TSO segment which
has not yet been completed, as indicated by seg_remain. Subtract those
seg_remain bytes from the mbuf length before computing the segment
offset.

Visual representation:

Before:
------------------------------------------------------------------
|    tso_    |    tso_    |    tso_    |    tso_    |    tso_    |
|    segsz   |    segsz   |    segsz   |    segsz   |    segsz   |
------------------------------------------------------------------
|           16K               |            16K               |   |
------------------------------------------------------------------

After:
------------------------------------------------------------------
|rem |    tso_    |    tso_    |    tso_    |    tso_    | seg_  |
|ain |    segsz   |    segsz   |    segsz   |    segsz   |offset |
------------------------------------------------------------------
|           16K               |            16K               |   |
------------------------------------------------------------------
                                                               ^
                                                        mbuf_remain

In this example, the original code would have found that tso_segsz
evenly divides mbuf_len, and that there is no tail to be considered
split across two mbufs. So the number of descriptors it would have found
would be:

  1 (for the headers) +
  0 (no segment offset) +
  0 (mbuf_remain cannot be less than 0)

For a total of one descriptor counted in num_descs for the start of
the current TSO segment.

Meanwhile, the reality would be:

  1 (for the headers) +
  1 (seg_offset != 0) +
  1 (mbuf_remain < seg_offset)

For a total of 3 descriptors counted against the descriptor limit.

This undercount can cause TSO segments to incorrectly cross up to two
extra descriptors more than the hardware limit, leading to the datapath
being disabled.

Fixes: be8f0eb81f98 ("net/gve: fix DQO TSO descriptor limit")
Cc: stable@dpdk.org

Signed-off-by: Joshua Washington <joshwash@google.com>
Reviewed-by: Praveen Kaligineedi <pkaligineedi@google.com>
Reviewed-by: Ankit Garg <nktgrg@google.com>
---
Depends-on: series-39707 ("net/gve: fix DQO Tx queue stalls")
---
 drivers/net/gve/gve_tx_dqo.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/gve/gve_tx_dqo.c b/drivers/net/gve/gve_tx_dqo.c
index 52fcdb285a..3a20109a5d 100644
--- a/drivers/net/gve/gve_tx_dqo.c
+++ b/drivers/net/gve/gve_tx_dqo.c
@@ -191,7 +191,7 @@ gve_tx_validate_descs(const struct gve_tx_queue *txq, struct rte_mbuf *tx_pkt,
 			return false;
 
 		if (seg_remain < mbuf_len) {
-			seg_offset = mbuf_len % tso_segsz;
+			seg_offset = (mbuf_len - seg_remain) % tso_segsz;
 			/* The MSS is bound from above by 9728B, so a
 			 * single TSO segment in the middle of an mbuf
 			 * will be part of at most two descriptors, and
-- 
2.56.0.307.g5fe7bd680c-goog