On Fri, Feb 23, 2018 at 6:51 PM, Andy Shevchenko <andy.shevchenko@xxxxxxxxx> wrote: > On Fri, Feb 23, 2018 at 6:41 PM, David Laight <David.Laight@xxxxxxxxxx> wrote: >> From: Arnd Bergmann >>> Sent: 23 February 2018 15:37 >>> >>> 32-bit architectures generally cannot use writeq(), so we now get a build >>> failure for the lpfc driver: >>> >>> drivers/scsi/lpfc/lpfc_sli.c: In function 'lpfc_sli4_wq_put': >>> drivers/scsi/lpfc/lpfc_sli.c:145:4: error: implicit declaration of function 'writeq'; did you mean >>> 'writeb'? [-Werror=implicit-function-declaration] >>> >>> Another problem here is that writing out actual data (unlike accessing >>> mmio registers) means we must write the data with the same endianess >>> that we have read from memory, but writeq() will perform byte swaps >>> and add barriers inbetween accesses as we do for registers. >>> >>> Using memcpy_toio() should do the right thing here, using register >>> sized stores with correct endianess conversion and barriers (i.e. none), >>> but on some architectures might fall back to byte-size access. >> ... >> >> Have you looked at the performance impact of this on x86? >> Last time I looked memcpy_toio() aliased directly to memcpy(). >> memcpy() is run-time patched between several different algorithms. >> On recent Intel cpus memcpy() is implemented as 'rep movsb' relying >> on the hardware to DTRT. >> For uncached accesses (typical for io) the 'RT' has to be byte transfers. >> So instead of the 8 byte transfers (on 64 bit) you get single bytes. >> This won't be what is intended! >> memcpy_toio() should probably use 'rep movsd' for the bulk of the transfer. > > Maybe I'm wrong but it uses movsq on 64-bit and movsl on 32-bit. > > The side-effect I referred previously is about tails, i.e. unaligned > bytes are transferred in portions > like > 7 on 64-bit will be 4 + 2 + 1, > 5 = 4 + 1 > etc > > Similar way on 32-bit. Same for leading bytes as well. arch/x86/lib/memcpy_64.S So, I *hope* that in the code in question there is no unaligned access is used. -- With Best Regards, Andy Shevchenko
