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Tsirkin" To: Eric Dumazet Cc: Linus Torvalds , Guenter Roeck , Xuan Zhuo , Linux Kernel Mailing List , Netdev Subject: Re: Linux 5.12-rc7 Message-ID: <20210413093606-mutt-send-email-mst@kernel.org> References: <20210412051445.GA47322@roeck-us.net> <20210413085538-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 13, 2021 at 03:33:40PM +0200, Eric Dumazet wrote: > On Tue, Apr 13, 2021 at 3:27 PM Eric Dumazet wrote: > > > > On Tue, Apr 13, 2021 at 2:57 PM Michael S. Tsirkin wrote: > > > > > > On Mon, Apr 12, 2021 at 06:47:07PM +0200, Eric Dumazet wrote: > > > > On Mon, Apr 12, 2021 at 6:31 PM Eric Dumazet wrote: > > > > > > > > > > On Mon, Apr 12, 2021 at 6:28 PM Linus Torvalds > > > > > wrote: > > > > > > > > > > > > On Sun, Apr 11, 2021 at 10:14 PM Guenter Roeck wrote: > > > > > > > > > > > > > > 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). > > > > */ > > > > > > Question: do we still want to do this for performance reasons? > > > We also have the hdr_len coming from the device which is > > > just skb_headlen on the host. > > > > Well, putting 20 bytes in skb->head will disable frag0 optimization. > > > > The change would only benefit to sh architecture :) > > > > About hdr_len, I suppose we could try it, with appropriate safety checks. > > I have added traces, hdr_len seems to be 0 with the qemu-system-sh4 I am using. > > Have I understood you correctly ? > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c > index 0824e6999e49957f7aaf7c990f6259792d42f32b..f024860f7dc260d4efbc35a3b8ffd358bd0da894 > 100644 > --- a/drivers/net/virtio_net.c > +++ b/drivers/net/virtio_net.c > @@ -399,9 +399,10 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi, > hdr_padded_len = sizeof(struct padded_vnet_hdr); > > /* hdr_valid means no XDP, so we can copy the vnet header */ > - if (hdr_valid) > + if (hdr_valid) { > memcpy(hdr, p, hdr_len); > - > + pr_err("hdr->hdr_len=%u\n", hdr->hdr.hdr_len); > + } > len -= hdr_len; > offset += hdr_padded_len; > p += hdr_padded_len; Depends on how you connect qemu on the host. It's filled by host tap, see virtio_net_hdr_from_skb. If you are using slirp that just zero-fills it. -- MST