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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