Re: [PATCH] arm64: Allow disabling of the compat vDSO

[Nick Desaulniers]

On Wed, Sep 25, 2019 at 10:08 AM Catalin Marinas
<catalin.marinas@xxxxxxx> wrote:
>
> This is just a temporary hiding of the issue, not a complete fix.

Yep.

> Vincenzo will do the fix later on.

Appreciated, I'm happy to help discuss, review, and test.

> > > - check whether COMPATCC is clang or not rather than CC_IS_CLANG, which
> > >   only checks the native compiler
> >
> > When cross compiling, IIUC CC_IS_CLANG is referring to CC which is the
> > cross compiler, which is different than HOSTCC which is the host
> > compiler.  HOSTCC is used mostly for things in scripts/ while CC is
> > used to compile a majority of the kernel in a cross compile.
>
> We need the third compiler here for the compat vDSO (at least with gcc),
> COMPATCC. I'm tempted to just drop the CONFIG_CROSS_COMPILE_COMPAT_VDSO
> altogether and only rely on a COMPATCC. This way we can add
> COMPATCC_IS_CLANG etc. in the Kconfig checks directly.

Oh, man, yeah 3 compilers in that case:
1. HOSTCC
2. CC
3. COMPATCC

>
> If clang can build both 32 and 64-bit with the same binary (just
> different options), we could maybe have COMPATCC default to CC and add a
> check on whether COMPATCC generates 32-bit binaries.

Cross compilation work differently between GCC and Clang from a
developers perspective. In GCC, at ./configure time you select which
architecture you'd like to cross compile for, and you get one binary
that targets one architecture.  You get a nice compiler that can do
mostly static dispatch at the cost of needing multiple binaries in
admittedly rare scenarios like the one we have here.  Clang defaults
to all backends enabled when invoking cmake (though there are options
to enable certain backends; Sony for instance enables only x86_64 for
their PS4 SDK (and thus I break their build frequently with my arm64
unit tests)).

Clang can do all of the above with one binary.  The codebase makes
heavy use of OOP+virtual dispatch to run ISA specific and general code
transformations (virtual dispatch is slower than static dispatch, but
relative to what the compiler is actually doing, I suspect the effects
are minimal. Folks are also heavily invested in researching
"devirtualization").  With one clang binary, I can do:

# implicitly uses the host's ISA, for me that's x86_64-linux-gnu
$ clang foo.c
$ clang -target aarch64-linux-gnu foo.c
$ clang -target arm-linux-gnueabi foo.c

Admittedly, it's not as simple for the kernel's case; the top level
Makefile sets some flags to support invoking Clang from a non-standard
location, and telling it where to find binutils because we can't
assemble the kernel with LLVM's substitute for GAS).
-- 
Thanks,
~Nick Desaulniers