Re: [dpdk-dev] [PATCH v3 03/10] net/enic: heed the requested max Rx packet size

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi everyone,

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ferruh Yigit
> Sent: Friday, March 9, 2018 3:04 PM
> To: John Daley <johndale@cisco.com>
> Cc: dev@dpdk.org; Hyong Youb Kim <hyonkim@cisco.com>
> Subject: Re: [dpdk-dev] [PATCH v3 03/10] net/enic: heed the requested max Rx packet size
> 
> On 3/8/2018 2:46 AM, John Daley wrote:
> > From: Hyong Youb Kim <hyonkim@cisco.com>
> >
> > Currently, enic completely ignores the requested max Rx packet size
> > (rxmode.max_rx_pkt_len). The desired behavior is that the NIC hardware
> > drops packets larger than the requested size, even though they are
> > still smaller than MTU.
> 
> Your description looks reasonable but is there reason of two levels of limits,
> max_rx_pkt_len and MTU, why not just use MTU. There is already a mail thread to
> clarify max_rx_pkt_len [1].
> 
> Is this work based on an application that uses max_rx_pkt_len and to make PMD
> compatible with that application? If so we can continue with patch, but if the
> patch is to implement DPDK properly I suggest postponing this until
> max_rx_pkt_len clarified.
> 
> [1]
> https://dpdk.org/ml/archives/dev/2018-March/092178.html

I think there are quite a lot apps these days that might rely on setting MTU via
rxmode.max_rx_pkt_len.
I think we need to support them till we (ever) deprecate rxmode.max_rx_pkt_len.
Konstantin

> 
> >
> > Cisco VIC does not have such a feature. But, we can accomplish a
> > similar (not same) effect by reducing the size of posted receive
> > buffers. Packets larger than the posted size get truncated, and the
> > receive handler drops them. This is also how the kernel enic driver
> > enforces the Rx side MTU.
> >
> > This workaround works only when scatter mode is *not* used. When
> > scatter is used, there is currently no way to support
> > rxmode.max_rx_pkt_len, as the NIC always receives packets up to MTU.
> >
> > For posterity, add a copious amount of comments regarding the
> > hardware's drop/receive behavior with respect to max/current MTU.
> >
> > Signed-off-by: Hyong Youb Kim <hyonkim@cisco.com>
> > Reviewed-by: John Daley <johndale@cisco.com>
> 
> <...>