Re: Linux 5.12-rc7

On Mon, Apr 12, 2021 at 6:31 PM Eric Dumazet [off-list ref] wrote:

On Mon, Apr 12, 2021 at 6:28 PM Linus Torvalds
[off-list ref] wrote:

quoted

On Sun, Apr 11, 2021 at 10:14 PM Guenter Roeck [off-list ref] wrote:

quoted

Qemu test results:
        total: 460 pass: 459 fail: 1
Failed tests:
        sh:rts7751r2dplus_defconfig:ata:net,virtio-net:rootfs

The failure bisects to commit 0f6925b3e8da ("virtio_net: Do not pull payload in
skb->head"). It is a spurious problem - the test passes roughly every other
time. When the failure is seen, udhcpc fails to get an IP address and aborts
with SIGTERM. So far I have only seen this with the "sh" architecture.

Hmm. Let's add in some more of the people involved in that commit, and
also netdev.

Nothing in there looks like it should have any interaction with
architecture, so that "it happens on sh" sounds odd, but maybe it's
some particular interaction with the qemu environment.

Yes, maybe.

I spent few hours on this, and suspect a buggy memcpy() implementation
on SH, but this was not conclusive.

By pulling one extra byte, the problem goes away.

Strange thing is that the udhcpc process does not go past sendto().

This is the patch working around the issue. Unfortunately I was not
able to root-cause it (I really suspect something on SH)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 0824e6999e49957f7aaf7c990f6259792d42f32b..fd890a951beea03bdf24406809042666eb972655

100644

--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -408,11 +408,17 @@ static struct sk_buff *page_to_skb(struct

virtnet_info *vi,

        /* Copy all frame if it fits skb->head, otherwise
         * we let virtio_net_hdr_to_skb() and GRO pull headers as needed.
+        *
+        * Apparently, pulling only the Ethernet Header triggers a bug
on qemu-system-sh4.
+        * Since GRO aggregation really cares of IPv4/IPv6, pull 20 bytes
+        * more to work around this bug : These 20 bytes can not belong
+        * to UDP/TCP payload.
+        * As a bonus, this makes GRO slightly faster for IPv4 (one less copy).
         */
        if (len <= skb_tailroom(skb))
                copy = len;
        else
-               copy = ETH_HLEN + metasize;
+               copy = ETH_HLEN + sizeof(struct iphdr) + metasize;
        skb_put_data(skb, p, copy);

        if (metasize) {