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Subject: Re: [PATCH net-next v2 6/9] net: macsec: hardware offloading
 infrastructure
To:     Andrew Lunn <andrew@lunn.ch>,
        Igor Russkikh <Igor.Russkikh@aquantia.com>
Cc:     Antoine Tenart <antoine.tenart@bootlin.com>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "sd@queasysnail.net" <sd@queasysnail.net>,
        "hkallweit1@gmail.com" <hkallweit1@gmail.com>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "thomas.petazzoni@bootlin.com" <thomas.petazzoni@bootlin.com>,
        "alexandre.belloni@bootlin.com" <alexandre.belloni@bootlin.com>,
        "allan.nielsen@microchip.com" <allan.nielsen@microchip.com>,
        "camelia.groza@nxp.com" <camelia.groza@nxp.com>,
        Simon Edelhaus <Simon.Edelhaus@aquantia.com>,
        Pavel Belous <Pavel.Belous@aquantia.com>
References: <20190808140600.21477-1-antoine.tenart@bootlin.com>
 <20190808140600.21477-7-antoine.tenart@bootlin.com>
 <e96fa4ae-1f2c-c1be-b2d8-060217d8e151@aquantia.com>
 <20190813085817.GA3200@kwain> <20190813131706.GE15047@lunn.ch>
 <2e3c2307-d414-a531-26cb-064e05fa01fc@aquantia.com>
 <20190813162823.GH15047@lunn.ch>
From:   Florian Fainelli <f.fainelli@gmail.com>
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Message-ID: <384841e8-694b-8f8b-ff91-396d2498170a@gmail.com>
Date:   Wed, 14 Aug 2019 16:28:49 -0700
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On 8/13/19 9:28 AM, Andrew Lunn wrote:
>> 1) With current implementation it's impossible to install SW macsec engine onto
>> the device which supports HW offload. That could be a strong limitation in
>> cases when user sees HW macsec offload is broken or work differently, and he/she
>> wants to replace it with SW one.
>> MACSec is a complex feature, and it may happen something is missing in HW.
>> Trivial example is 256bit encryption, which is not always a musthave in HW
>> implementations.
> 
> Ideally, we want the driver to return EOPNOTSUPP if it does not
> support something and the software implement should be used.
> 
> If the offload is broken, we want a bug report! And if it works
> differently, it suggests there is also a bug we need to fix, or the
> standard is ambiguous.
> 
> It would also be nice to add extra information to the netlink API to
> indicate if HW or SW is being used. In other places where we offload
> to accelerators we have such additional information.

Igor's point is entirely valid in that you should allow both offload to
HW for what is possible and offload to a software implementation for
what is not supported in HW.

Let's an example that is hopefully more familiar to the people in this
thread. Consider a NIC that can do single VLAN tag offload on xmit (or
receive, does not matter), and you find yourself using a double VLAN tag
configuration. You would create a first VLAN stacked network device
which is fully offloaded onto the underlying NIC, and a second VLAN
stacked network device on top of the first once which is not offloaded.

The way I would imagine a MACsec offload is kind of similar here, in
that it would be a macsec virtual network device on top of an underlying
physical device. If no offload is possible, the virtual network device's
xmit/receive path is used. If the NIC driver can offload it, it does
not. How it does it, whether at the MAC/DMA level, or at the PHY level
can be a check added at the appropriate level.
-- 
Florian