[PATCH v3] vhost: compilation fix for GCC-12

[Amit Prakash Shukla <amitprakashs@marvell.com>]

GCC-12 complains about the possible use of un-initialized array. At
compile time it seems like it is not able to evaluate the size as it
involves run-time variable and at compile time it seems like gcc assumes
value of "size" variable to be zero which makes gcc-12 to jump the while
loop.
"size = pkt->pkt_len + sizeof(struct virtio_net_hdr_mrg_rxbuf);"

As part of the fix, "while (){}" is replaced by "do {} while()" which make
the compiler to generate a code in which buf_vec will never be used
un-initialized.

../lib/vhost/virtio_net.c:941:35: error:
		'buf_vec[0].buf_len' may be used uninitialized
		[-Werror=maybe-uninitialized]
  941 |         buf_len = buf_vec[vec_idx].buf_len;
      |                   ~~~~~~~~~~~~~~~~^~~~~~~~
../lib/vhost/virtio_net.c: In function 'virtio_dev_rx_packed':
../lib/vhost/virtio_net.c:1285:27: note: 'buf_vec' declared here
 1285 |         struct buf_vector buf_vec[BUF_VECTOR_MAX];
      |                           ^~~~~~~
cc1: all warnings being treated as errors

Fixes: 93520085efda ("vhost: add packed ring single enqueue")
Cc: stable@dpdk.org

Signed-off-by: Amit Prakash Shukla <amitprakashs@marvell.com>
---
v2:
- Changes for code review suggestion

v3:
- Added a description

 lib/vhost/virtio_net.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
index b3d954aab4..9b77d3d10f 100644
--- a/lib/vhost/virtio_net.c
+++ b/lib/vhost/virtio_net.c
@@ -1069,7 +1069,7 @@ vhost_enqueue_single_packed(struct virtio_net *dev,
 	else
 		max_tries = 1;
 
-	while (size > 0) {
+	do {
 		/*
 		 * if we tried all available ring items, and still
 		 * can't get enough buf, it means something abnormal
@@ -1097,7 +1097,7 @@ vhost_enqueue_single_packed(struct virtio_net *dev,
 		avail_idx += desc_count;
 		if (avail_idx >= vq->size)
 			avail_idx -= vq->size;
-	}
+	} while (size > 0);
 
 	if (mbuf_to_desc(dev, vq, pkt, buf_vec, nr_vec, num_buffers, false) < 0)
 		return -1;
@@ -1574,7 +1574,7 @@ vhost_enqueue_async_packed(struct virtio_net *dev,
 	else
 		max_tries = 1;
 
-	while (size > 0) {
+	do {
 		/*
 		 * if we tried all available ring items, and still
 		 * can't get enough buf, it means something abnormal
@@ -1601,7 +1601,7 @@ vhost_enqueue_async_packed(struct virtio_net *dev,
 		avail_idx += desc_count;
 		if (avail_idx >= vq->size)
 			avail_idx -= vq->size;
-	}
+	} while (size > 0);
 
 	if (unlikely(mbuf_to_desc(dev, vq, pkt, buf_vec, nr_vec, *nr_buffers, true) < 0))
 		return -1;
-- 
2.25.1