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