netdev - Re: [PATCH 1/2] [for 4.13] net: qcom/emac: disable flow control autonegotiation by default

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 2 Aug 2017 10:08:35 -0500
From:   Timur Tabi <timur@...eaurora.org>
To:     David Laight <David.Laight@...LAB.COM>,
        "David S. Miller" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/2] [for 4.13] net: qcom/emac: disable flow control
 autonegotiation by default

On 08/02/2017 09:51 AM, David Laight wrote:
> Sending pause frames just tells the adjacent switch not to send you packets
> (because you'll discard them).
> Since the idea is to avoid the discards, the switch will buffer the
> packets it would have sent.
> The buffers in the switch then fill up with packets it isn't sending you.

I was under the impression that the switch forwards the pause frames to 
other devices, so that the transmitting NIC can stop sending the data, 
but your explanation makes a lot more sense.  If the EMAC never stops 
sending pause frames, then the switch's buffers will fill up, disabling 
all other devices.  If the switch does not have per-port buffers, then 
it makes sense when the buffer is full, it blocks all ports.

> The switch then runs out of buffers, it has 2 choices:
> 1) Throw the packets away.
> 2) Send 'pause' frames to the sources.
> If it sends 'pause' frames the entire network will very quickly lock up.
> If it discards the packets they might as well have been discarded by the
> receiving MAC.
> 
> Doesn't this mean that pause frames are 99.999% useless??

Pause frames are intended for situations where the receiving CPU is 
temporarily overwhelmed and just needs a second or two to resume 
processing incoming packets.  That makes sense on a dinky single-core 
32-bit CPU.

-- 
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm
Technologies, Inc.  Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.