On Thu, Oct 04, 2018 at 07:43:59PM +0200, Ard Biesheuvel wrote: > (+ Arnd, Russell, Catalin, Will) > > On 4 October 2018 at 19:36, Ben Hutchings <ben.hutchi...@codethink.co.uk> > wrote: > > NET_IP_ALIGN is supposed to be defined as 0 if DMA writes to an > > unaligned buffer would be more expensive than CPU access to unaligned > > header fields, and otherwise defined as 2. > > > > Currently only ppc64 and x86 configurations define it to be 0. > > However several other architectures (conditionally) define > > CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS, which seems to imply that > > NET_IP_ALIGN should be 0. > > > > Remove the overriding definitions for ppc64 and x86 and define > > NET_IP_ALIGN solely based on CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS. > > > > Signed-off-by: Ben Hutchings <ben.hutchi...@codethink.co.uk> > > While this makes sense for arm64, I don't think it is appropriate for > ARM per se. > > The unusual thing about ARM is that some instructions require 32-bit > alignment even when CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is set, > (i.e., load/store multiple, load/store double), and we rely on > alignment fixups done by the kernel to deal with the fallout if such > instructions happen to be used on unaligned quantities (Russell, > please correct me if this is inaccurate)
Correct, and we do have some assembly that use ldmia in the net code (eg, for checksum calculation.) Having NET_IP_ALIGN be 0 on ARM coupled with a network adapter that doesn't do its own checksumming would mean every non-hw-checksummed IP packet hitting the alignment fixup - and not just once per packet. So it's likely that this change could provoke reports of performance regressions for ARM. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up
