Date: Tue, 6 Dec 2016 10:46:25 -0800 From: Peter Grehan <grehan@freebsd.org> To: =?UTF-8?B?Sm9zw6kgR2FyY8OtYSBKdWFuaW5v?= <jjuanino@gmail.com> Cc: freebsd-virtualization@freebsd.org Subject: Re: bhyve: cannot send jumbo frames from linux or freebsd guest Message-ID: <414db7f0-ea86-2080-4179-eb77a18a67a5@freebsd.org> In-Reply-To: <CAAVO5%2BJnk1MepU7mBdQiWsRJf4FF0EuheaERGCHJJw3_pgG_dA@mail.gmail.com> References: <CAAVO5%2BJnk1MepU7mBdQiWsRJf4FF0EuheaERGCHJJw3_pgG_dA@mail.gmail.com>
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Hi, > * From FreeBSD guest: > # tcpdump -vvn -i vtnet1 & > tcpdump: listening on vtnet1, link-type EN10MB (Ethernet), capture > size 262144 bytes > # ping -c 1 -D -s 1995 192.168.253.1 > PING 192.168.253.1 (192.168.253.1): 1995 data bytes > 23:08:46.808187 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto > ICMP (1), length 2023) > 192.168.253.4 > 192.168.253.1: ICMP echo request, id 58116, seq 0, > length 2003 > 23:08:46.808430 IP truncated-ip - 1 bytes missing! (tos 0x0, ttl 64, > id 0, offset 0, flags [DF], proto ICMP (1), length 2023) > 192.168.253.1 > 192.168.253.4: ICMP echo reply, id 58116, seq 0, length 2003 This looks like a bug in the FreeBSD virtio-net driver bpf code, since the host dump shows those sized packets going out correctly. > Could anyone explain why I cannot send jumbo frames from Linux and FreeBSD > guests, but I *can* send them from Windows guest? Am I missing o > missunderstanding something? Thanks in advanced, any comment o suggestion will > be wellcome. There is currently a bug in bhyve with handling of received frames. The virtio implementation advertises 'rx-merge' capability, which allows frames to span buffers that have been queued by the guest. However, bhyve currently doesn't copy the buffer across multiple buffers. I suspect that the Windows virtio guest driver doesn't use rx-merge but instead uses 'indirect descriptors', which is why it works. A short-term workaround for FreeBSD and Linux guests is to modify bhyve to not advertise the rx-merge capability: usr.sbin/bhyve/pci_virtio_net.c #define VTNET_S_HOSTCAPS \ - ( VIRTIO_NET_F_MAC | VIRTIO_NET_F_MRG_RXBUF | VIRTIO_NET_F_STATUS | \ + ( VIRTIO_NET_F_MAC | VIRTIO_NET_F_STATUS | \ (Note that this doesn't work with Net/OpenBSD, where jumbo frames are only supported via rx-merge.) I hope to have a fix for the rx-merge issue in the near future - it's a more efficient way of handling jumbo frames than indirect descriptors. later, Peter.
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