Hi!
On Mon, Mar 15, 2021 at 02:41:59PM +1100, Michael Ellerman wrote:
> The ".machine" directive allows changing the machine for which code is
> being generated. It's equivalent to passing an -mcpu option on the
> command line.
>
> Although it can be useful, it's generally a bad idea because it adds
> another way to influence code generation separate from the flags
> passed via the build system. ie. if we need to build different pieces
> of code with different flags we should do that via our Makefiles, not
> using ".machine".
It does not influence code generation. It says which instructions are
valid, instead. There are a few cases where the same mnemonic will
generate a different binary encoding depending on machine selected,
maybe you mean that?
It is *normal* to use .machine push/pop and a specific .machine around
instructions that require a machine other than what you are building
for. The compiler does this itself, and it is the recommended way to
use "foreign" instructions in inline assembler.
That said...
> However as best as I can tell the ".machine" directive in
> trampoline_64.S is not necessary at all.
>
> It was added in commit 0d97631392c2 ("powerpc: Add purgatory for
> kexec_file_load() implementation."), which created the file based on
> the kexec-tools purgatory. It may be/have-been necessary in the
> kexec-tools version, but we have a completely different build system,
> and we already pass the desired CPU flags, eg:
>
> gcc ... -m64 -Wl,-a64 -mabi=elfv2 -Wa,-maltivec -Wa,-mpower4 -Wa,-many
> ... arch/powerpc/purgatory/trampoline_64.S
>
> So drop the ".machine" directive and rely on the assembler flags.
> - .machine ppc64
Please make sure to test this on a big endian config.
A ppc64le-linux assembler defaults to power8. A ppc64-linux assembler
defaults to power3 (that is the same as .machine ppc64). Or maybe it
makes it power4? I get lost :-)
It certainly *should* work, but, test please :-)
(And with a *default* powerpc64-linux config, not one that defaults to
power7 or power8 or similar! Arnd's toolchains at
<https://mirrors.edge.kernel.org/pub/tools/crosstool/>
are fine for this.)
Reviewed-by: Segher Boessenkool <seg...@kernel.crashing.org>
Segher
