netdev - Re: [net-next v4 00/15] Add mlx5 subfunction support

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Date:   Thu, 17 Dec 2020 20:55:10 -0700
From:   David Ahern <dsahern@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        Saeed Mahameed <saeed@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Leon Romanovsky <leonro@...dia.com>,
        Netdev <netdev@...r.kernel.org>, linux-rdma@...r.kernel.org,
        David Ahern <dsahern@...nel.org>,
        Jacob Keller <jacob.e.keller@...el.com>,
        Sridhar Samudrala <sridhar.samudrala@...el.com>,
        "Ertman, David M" <david.m.ertman@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kiran Patil <kiran.patil@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [net-next v4 00/15] Add mlx5 subfunction support

On 12/17/20 8:11 PM, Alexander Duyck wrote:
> On Thu, Dec 17, 2020 at 5:30 PM David Ahern <dsahern@...il.com> wrote:
>>
>> On 12/16/20 3:53 PM, Alexander Duyck wrote:
>>> The problem in my case was based on a past experience where east-west
>>> traffic became a problem and it was easily shown that bypassing the
>>> NIC for traffic was significantly faster.
>>
>> If a deployment expects a lot of east-west traffic *within a host* why
>> is it using hardware based isolation like a VF. That is a side effect of
>> a design choice that is remedied by other options.
> 
> I am mostly talking about this from past experience as I had seen a
> few instances when I was at Intel when it became an issue. Sales and
> marketing people aren't exactly happy when you tell them "don't sell
> that" in response to them trying to sell a feature into an area where

that's a problem engineers can never solve...

> it doesn't belong. Generally they want a solution. The macvlan offload
> addressed these issues as the replication and local switching can be
> handled in software.

well, I guess almost never. :-)

> 
> The problem is PCIe DMA wasn't designed to function as a network
> switch fabric and when we start talking about a 400Gb NIC trying to
> handle over 256 subfunctions it will quickly reduce the
> receive/transmit throughput to gigabit or less speeds when
> encountering hardware multicast/broadcast replication. With 256
> subfunctions a simple 60B ARP could consume more than 19KB of PCIe
> bandwidth due to the packet having to be duplicated so many times. In
> my mind it should be simpler to simply clone a single skb 256 times,
> forward that to the switchdev ports, and have them perform a bypass
> (if available) to deliver it to the subfunctions. That's why I was
> thinking it might be a good time to look at addressing it.
> 

east-west traffic within a host is more than likely the same tenant in
which case a proper VPC is a better solution than the s/w stack trying
to detect and guess that a bypass is needed. Guesses cost cycles in the
fast path which is a net loss - and even more so as speeds increase.