[PATCH] virtio_net: lower limit on buffer size

["Michael S. Tsirkin" <mst@redhat.com>]

commit d85b758f72b0 "virtio_net: fix support for small rings"
was supposed to increase the buffer size for small rings
but had an unintentional side effect of decreasing
it for large rings. This seems to break some setups -
it's not yet clear why, but increasing buffer size
back to what it was before helps.

Fixes: d85b758f72b0 "virtio_net: fix support for small rings"
Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Reported-by: "J. Bruce Fields" <bfields@fieldses.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

Can reporters please confirm whether the following helps?
If it does I think we should queue it even though I expect I'll need to
investigate the exact reasons for the failure down the road - probably
a hypervisor bug.

 drivers/net/virtio_net.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 87b5c20..60abb5d 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -842,7 +842,7 @@ static unsigned int get_mergeable_buf_len(struct receive_queue *rq,
 	unsigned int len;
 
 	len = hdr_len + clamp_t(unsigned int, ewma_pkt_len_read(avg_pkt_len),
-				rq->min_buf_len - hdr_len, PAGE_SIZE - hdr_len);
+				rq->min_buf_len, PAGE_SIZE - hdr_len);
 	return ALIGN(len, L1_CACHE_BYTES);
 }
 
@@ -2039,7 +2039,8 @@ static unsigned int mergeable_min_buf_len(struct virtnet_info *vi, struct virtqu
 	unsigned int buf_len = hdr_len + ETH_HLEN + VLAN_HLEN + packet_len;
 	unsigned int min_buf_len = DIV_ROUND_UP(buf_len, rq_size);
 
-	return max(min_buf_len, hdr_len);
+	return max(max(min_buf_len, hdr_len) - hdr_len,
+		   (unsigned int)GOOD_PACKET_LEN);
 }
 
 static int virtnet_find_vqs(struct virtnet_info *vi)
-- 
MST