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

[Jeremy Kerr <jk@codeconstruct.com.au>]

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