[Bug 194314] [ixgbe] driver makes some dangerous assumptions with struct mbuf sizing with IXGBE_RX_COPY_LEN

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=194314

--- Comment #10 from Garrett Cooper <ngie at FreeBSD.org> ---
(In reply to Eric Joyner from comment #8)
> So it sounds like we may just need to change IXGBE_RX_COPY_LEN from a fixed
> value of 160 to something that's calculated based on the length of struct
> m_hdr and struct m_pkthdr combined.
> 
> I think they got 160 by adding together the size of the two header structs
> (88 in FreeBSD amd64), adding some number (8) to make that size divisible by
> 32 (96 % 32 == 0), and then subtracted that from MSIZE (256) to get
> IXGBE_RX_COPY_LEN (160), while keeping the padding added as
> IXGBE_RX_COPY_ALIGN.

A couple questions then:

- What architectures is this driver supported on?
- Is this optimization only valid for amd64?
- Why 32?

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