Messages in this thread | | | Date | Wed, 20 Sep 2017 16:41:43 +0100 | From | Russell King - ARM Linux <> | Subject | Re: [PATCH] ARM: unaligned.h: Use an arch-specific version |
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On Wed, Sep 20, 2017 at 08:26:09AM -0700, Ard Biesheuvel wrote: > Hi Romain, > > On 20 September 2017 at 08:18, Romain Izard <romain.izard.pro@gmail.com> wrote: > > For the 32-bit ARM architecture, unaligned access support is variable. > > On a chip without a MMU, an unaligned access returns a rotated data word > > and must be avoided. > > > > When a MMU is available, it can be trapped. On ARMv6 or ARMv7, it can also > > be handled by the hardware, depending on the type of access instruction. > > Unaligned access of 32 bits or less are corrected, while larger access > > provoke a trap. > > > > Unfortunately, the compiler is able to merge two 32-bit access that > > would generate a LDR instruction, that works on unaligned access, into a > > single LDM access that will not work. This is not a common situation, > > but it has been observed in the LZ4 decompression code. > > > > To prevent this type of optimization, it is necessary to change the > > explicit accessors for unaligned addresses from those defined in the > > access_ok.h header, to those defined in the packed_struct.h header. > > > > Add an arch-specific header to ARM, to retain other optimizations that > > rely on HAVE_EFFICIENT_UNALIGNED_ACCESS, while making sure that access > > that explicitly rely on the unaligned accessors are correctly handled by > > the compiler. > > > > Signed-off-by: Romain Izard <romain.izard.pro@gmail.com> > > --- > > > > If access_ok.h has been observed to produce different output from the > struct versions (using any compiler), I guess we cannot simply change > the asm-generic default and expect everybody to be ok with that. So I > agree this is the most appropriate course of action.
However, what effect does this have on the code generated for the rest of the kernel?
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