On Monday, November 28, 2016 4:14:04 PM EST Alex Sidorenko wrote:
> On Monday, November 28, 2016 3:54:59 PM EST David Miller wrote:
> > From: Alex Sidorenko <alexandre.sidore...@hpe.com>
> > Date: Mon, 28 Nov 2016 15:49:26 -0500
> >
> > > Now the question is whether is is OK to have icsk->icsk_ack.rcv_mss
> > > larger than MTU.
> >
> > It absolutely is not OK.
> >
> > If VMWare wants to receive large frames for batching purposes it must
> > use GRO or similar to achieve that, not just send vanilla frames into
> > the stack which are larger than the device MTU.
> >
>
> As VMWare's vmxnet3 driver is open-sourced and part of generic kernel, do you
> think the problem is in that driver or elsewhere? I looked at vmxnet3 sources
> and see that it uses LRO/GRO subroutines. Unfortunately, I don't understand
> its logic enough to see whether they are doing anything incorrectly.
I think this has been already fixed in recent versions of vmxnet3 driver (but
not in RHEL6). VMWare/ESX can pass us aggregated large SKBs indeed (> MTU) if
LRO is enabled, but the driver takes care of that in vmxnet3_rq_rx_complete():
} else if (segCnt != 0 || skb->len > mtu) {
u32 hlen;
hlen = vmxnet3_get_hdr_len(adapter, skb,
(union Vmxnet3_GenericDesc *)rcd);
if (hlen == 0)
goto not_lro;
skb_shinfo(skb)->gso_type =
rcd->v4 ? SKB_GSO_TCPV4 : SKB_GSO_TCPV6;
if (segCnt != 0) {
skb_shinfo(skb)->gso_segs = segCnt;
skb_shinfo(skb)->gso_size =
DIV_ROUND_UP(skb->len -
hlen, segCnt);
} else {
skb_shinfo(skb)->gso_size = mtu - hlen;
}
}
So if packets have been aggregated,
u8 segCnt; /* Number of aggregated packets */
we compute gso_size by dividing large skb->len by the number.
I still like Marcelo's idea of printing a warning when icsk->icsk_ack.rcv_mss
looks unreasonable, should really help with detecting buggy drivers.
