[U-Boot] [PATCH] Use packed structures for networking

[Tom Rini]

On Sat, Jul 29, 2017 at 12:53:36PM +0300, Denis Pynkin wrote:
> 28.07.2017 00:26, Joe Hershberger wrote:
> 
> >>>PXE boot is broken with GCC 7.1 due option '-fstore-merging' enabled
> >>>by default for '-O2':
> 
> >>So, what I've been wondering, and others have poked me about (who can
> >>chime in if they like), how is the kernel not also tripping over this?
> >
> >Great question.
> 
> Believe kernel do not work in single static buffer for the whole
> packet as u-boot do.
> 
> Problem is in ethernet header structure with size ETHER_HDR_SIZE=14,
> so IP and BOOTP headers are not aligned to 4 bytes.
> 
> Let's take bootp.c code as example:
> 
> 755     bp = (struct bootp_hdr *)pkt;
> 756     bp->bp_op = OP_BOOTREQUEST;
> 757     bp->bp_htype = HWT_ETHER;
> 758     bp->bp_hlen = HWL_ETHER;
> 759     bp->bp_hops = 0;
> 
> Without '-fstore-merging' or with enabled 'packed' structures
> assembler code looks like:
>   6a:   77a3          strb    r3, [r4, #30]
>   6c:   f884 b01c     strb.w  fp, [r4, #28]
>   70:   f884 b01d     strb.w  fp, [r4, #29]
>   74:   77e5          strb    r5, [r4, #31]
> 
> but with option '-fstore-merging' all these instructions merged into
> single instruction:
>   62:   61e3          str     r3, [r4, #28]
> 
> and store address is not aligned to 32b boundary here, so it results
> to 'data abort'.
> 
> I see some possible solutions here:
> - add option '-fno-store-merging' -- works, I test it already
> - use packed structures -- since there are some unsafe code in sources
> - rewrite networking part to work with any protocol separately and
> merge headers and data only on send.
> 
> Even simple reordering of header flags initialization helps here,
> but it looks like a bit of black magic in code.
> 
> PS forgot to mention in patch description -- '-Os' includes
> '-fstore-merging' option as well for gcc 7.1.

Thanks for explaining.  My inclination is that we should have a packed
change in for -rc1.  Philipp, have you had a chance to edit
doc/README.unaligned-memory-access.txt to your liking?  Thanks!

-- 
Tom
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