netdev - Re: [PATCH net-next v6] mctp: Implement extended addressing

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Message-ID: <cbf0fd9611c76e557b759ecbecf6bcf712b44f55.camel@codeconstruct.com.au>
Date:   Tue, 26 Oct 2021 19:34:16 +0800
From:   Jeremy Kerr <jk@...econstruct.com.au>
To:     David Laight <David.Laight@...LAB.COM>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Matt Johnston <matt@...econstruct.com.au>,
        Eugene Syromiatnikov <esyr@...hat.com>
Subject: Re: [PATCH net-next v6] mctp: Implement extended addressing

Hi David,

> > +struct sockaddr_mctp_ext {
> > +       struct sockaddr_mctp    smctp_base;
> > +       int                     smctp_ifindex;
> > +       __u8                    smctp_halen;
> > +       __u8                    __smctp_pad0[3];
> > +       __u8                    smctp_haddr[MAX_ADDR_LEN];
> > +};
> 
> You'd be better off 8-byte aligning smctp_haddr.
> I also suspect that always copying the 32 bytes will be faster
> and generate less code than the memset() + memcpy().

The padding here is more to avoid layout variations between ABIs
rather than performance.

The largest current hardware address size that we need (for the i2c
transport) is... 1 byte. If we were to implement the PCIe VDM binding
for MCTP that'd then be the largest, now at 2 bytes. If anyone's crazy
enough to do MCTP over ethernet, we're still only at 6.

So, we'll be a long way off needing to optimise for 8-byte aligned
accesses here; I don't think the extra padding would be worth it.

Cheers,


Jeremy