Messages in this thread | | | Subject | Re: [PATCH 1/2] ARM: kvm: fix building with gcc-8 | From | Robin Murphy <> | Date | Fri, 2 Feb 2018 16:30:49 +0000 |
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On 02/02/18 16:29, Arnd Bergmann wrote: > On Fri, Feb 2, 2018 at 5:23 PM, Robin Murphy <robin.murphy@arm.com> wrote: >> On 02/02/18 15:55, Robin Murphy wrote: >>> >>> On 02/02/18 15:07, Arnd Bergmann wrote: >>>> >>>> In banked-sr.c, we use a top-level '__asm__(".arch_extension virt")' >>>> statement to allow compilation of a multi-CPU kernel for ARMv6 >>>> and older ARMv7-A that don't normally support access to the banked >>>> registers. >>>> >>>> This is considered to be a programming error by the gcc developers >>>> and will no longer work in gcc-8, where we now get a build error: >>>> >>>> /tmp/cc4Qy7GR.s:34: Error: Banked registers are not available with this >>>> architecture. -- `mrs r3,SP_usr' >>>> /tmp/cc4Qy7GR.s:41: Error: Banked registers are not available with this >>>> architecture. -- `mrs r3,ELR_hyp' >>>> /tmp/cc4Qy7GR.s:55: Error: Banked registers are not available with this >>>> architecture. -- `mrs r3,SP_svc' >>>> /tmp/cc4Qy7GR.s:62: Error: Banked registers are not available with this >>>> architecture. -- `mrs r3,LR_svc' >>>> /tmp/cc4Qy7GR.s:69: Error: Banked registers are not available with this >>>> architecture. -- `mrs r3,SPSR_svc' >>>> /tmp/cc4Qy7GR.s:76: Error: Banked registers are not available with this >>>> architecture. -- `mrs r3,SP_abt' >>>> >>>> Passign the '-march-armv7ve' flag to gcc works, and is ok here, because >>>> we know the functions won't ever be called on pre-ARMv7VE machines. >>>> Unfortunately, older compiler versions (4.8 and earlier) do not >>>> understand >>>> that flag, so we still need to keep the asm around. >>>> >>>> Backporting to stable kernels (4.6+) is needed to allow those to be built >>>> with future compilers as well. >>> >>> >>> Is "-Wa,arch=armv7-a+virt" (as we appear to do for a couple of files >>> already) viable as a possibly cleaner alternative, or is GCC itself now >>> policing the contents of inline asms? >> >> >> In fact, looking at the binutils history, any version capable of assembling >> this file should understand that (modulo my typo), so hopefully it ought to >> be feasible to replace these global asms with assembler flags entirely. > > No, this only works for .S files, not .c, since gcc starts the output with > an explicit .arch setting that overrides the command line. I think this > was done intentionally to prevent such a hack from working, and have > more reliable checks on the validity of the assembler instruction in > inline asm statements (which we try to circumvent here).
Ah, I see, that is unfortunate. Thanks for clarifying.
Robin.
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