Re: [PATCH 1/2] ARM: kvm: fix building with gcc-8

[Robin Murphy]

On 02/02/18 16:29, Arnd Bergmann wrote:
> On Fri, Feb 2, 2018 at 5:23 PM, Robin Murphy <robin.murphy@xxxxxxx> 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.