From: Johannes Berg In many cases, especially in networking, it can be beneficial to know at compile time whether the architecture can do unaligned accesses efficiently. This patch introduces a new Kconfig symbol HAVE_EFFICIENT_UNALIGNED_ACCESS for that purpose and adds it to the powerpc and x86 architectures. Also add some documentation about alignment and networking, and especially one intended use of this symbol. Signed-off-by: Johannes Berg Acked-by: Sam Ravnborg Acked-by: Ingo Molnar [x86 architecture part] Cc: Signed-off-by: Andrew Morton --- Documentation/unaligned-memory-access.txt | 32 ++++++++++++++++++-- arch/Kconfig | 19 +++++++++++ arch/powerpc/Kconfig | 1 arch/x86/Kconfig | 1 4 files changed, 50 insertions(+), 3 deletions(-) diff -puN Documentation/unaligned-memory-access.txt~introduce-have_efficient_unaligned_access-kconfig-symbol Documentation/unaligned-memory-access.txt --- a/Documentation/unaligned-memory-access.txt~introduce-have_efficient_unaligned_access-kconfig-symbol +++ a/Documentation/unaligned-memory-access.txt @@ -218,9 +218,35 @@ If use of such macros is not convenient, where the source or destination (or both) are of type u8* or unsigned char*. Due to the byte-wise nature of this operation, unaligned accesses are avoided. + +Alignment vs. Networking +======================== + +On architectures that require aligned loads, networking requires that the IP +header is aligned on a four-byte boundary to optimise the IP stack. For +regular ethernet hardware, the constant NET_IP_ALIGN is used. On most +architectures this constant has the value 2 because the normal ethernet +header is 14 bytes long, so in order to get proper alignment one needs to +DMA to an address which can be expressed as 4*n + 2. One notable exception +here is powerpc which defines NET_IP_ALIGN to 0 because DMA to unaligned +addresses can be very expensive and dwarf the cost of unaligned loads. + +For some ethernet hardware that cannot DMA to unaligned addresses like +4*n+2 or non-ethernet hardware, this can be a problem, and it is then +required to copy the incoming frame into an aligned buffer. Because this is +unnecessary on architectures that can do unaligned accesses, the code can be +made dependent on CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS like so: + +#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS + skb = original skb +#else + skb = copy skb +#endif + -- -Author: Daniel Drake +Authors: Daniel Drake , + Johannes Berg With help from: Alan Cox, Avuton Olrich, Heikki Orsila, Jan Engelhardt, -Johannes Berg, Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock, -Uli Kunitz, Vadim Lobanov +Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock, Uli Kunitz, +Vadim Lobanov diff -puN arch/Kconfig~introduce-have_efficient_unaligned_access-kconfig-symbol arch/Kconfig --- a/arch/Kconfig~introduce-have_efficient_unaligned_access-kconfig-symbol +++ a/arch/Kconfig @@ -27,6 +27,25 @@ config KPROBES for kernel debugging, non-intrusive instrumentation and testing. If in doubt, say "N". +config HAVE_EFFICIENT_UNALIGNED_ACCESS + def_bool n + help + Some architectures are unable to perform unaligned accesses + without the use of get_unaligned/put_unaligned. Others are + unable to perform such accesses efficiently (e.g. trap on + unaligned access and require fixing it up in the exception + handler.) + + This symbol should be selected by an architecture if it can + perform unaligned accesses efficiently to allow different + code paths to be selected for these cases. Some network + drivers, for example, could opt to not fix up alignment + problems with received packets if doing so would not help + much. + + See Documentation/unaligned-memory-access.txt for more + information on the topic of unaligned memory accesses. + config KRETPROBES def_bool y depends on KPROBES && HAVE_KRETPROBES diff -puN arch/powerpc/Kconfig~introduce-have_efficient_unaligned_access-kconfig-symbol arch/powerpc/Kconfig --- a/arch/powerpc/Kconfig~introduce-have_efficient_unaligned_access-kconfig-symbol +++ a/arch/powerpc/Kconfig @@ -109,6 +109,7 @@ config PPC select HAVE_FTRACE select HAVE_IDE select HAVE_IOREMAP_PROT + select HAVE_EFFICIENT_UNALIGNED_ACCESS select HAVE_KPROBES select HAVE_KRETPROBES select HAVE_LMB diff -puN arch/x86/Kconfig~introduce-have_efficient_unaligned_access-kconfig-symbol arch/x86/Kconfig --- a/arch/x86/Kconfig~introduce-have_efficient_unaligned_access-kconfig-symbol +++ a/arch/x86/Kconfig @@ -28,6 +28,7 @@ config X86 select HAVE_FTRACE select HAVE_KVM if ((X86_32 && !X86_VOYAGER && !X86_VISWS && !X86_NUMAQ) || X86_64) select HAVE_ARCH_KGDB if !X86_VOYAGER + select HAVE_EFFICIENT_UNALIGNED_ACCESS config ARCH_DEFCONFIG string _