Re: [PATCH] arm64: LLVMLinux: Fix inline arm64 assembly for use with clang

[Ard Biesheuvel]

On 10 September 2014 19:38, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Mon, Sep 08, 2014 at 10:30:51AM +0100, Olof Johansson wrote:
>> On Fri, Sep 05, 2014 at 04:24:20PM -0700, behanw@xxxxxxxxxxxxxxxxxx wrote:
>> > From: Mark Charlebois <charlebm@xxxxxxxxx>
>> >
>> > Fix variable types for 64-bit inline assembly.
>> >
>> > This patch now works with both gcc and clang.
>> >
>> > Signed-off-by: Mark Charlebois <charlebm@xxxxxxxxx>
>> > Signed-off-by: Behan Webster <behanw@xxxxxxxxxxxxxxxxxx>
>> > ---
>> >  arch/arm64/include/asm/arch_timer.h | 26 +++++++++++++++-----------
>> >  arch/arm64/include/asm/uaccess.h    |  2 +-
>> >  arch/arm64/kernel/debug-monitors.c  |  8 ++++----
>> >  arch/arm64/kernel/perf_event.c      | 34 +++++++++++++++++-----------------
>> >  arch/arm64/mm/mmu.c                 |  2 +-
>> >  5 files changed, 38 insertions(+), 34 deletions(-)
>> >
>> > diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
>> > index 9400596..c1f87e0 100644
>> > --- a/arch/arm64/include/asm/arch_timer.h
>> > +++ b/arch/arm64/include/asm/arch_timer.h
>> > @@ -37,19 +37,23 @@ void arch_timer_reg_write_cp15(int access, enum arch_timer_reg reg, u32 val)
>> >     if (access == ARCH_TIMER_PHYS_ACCESS) {
>> >             switch (reg) {
>> >             case ARCH_TIMER_REG_CTRL:
>> > -                   asm volatile("msr cntp_ctl_el0,  %0" : : "r" (val));
>> > +                   asm volatile("msr cntp_ctl_el0,  %0"
>> > +                           : : "r" ((u64)val));
>>
>> Ick. Care to elaborate in the patch description why this is needed with
>> LLVM? It's really messy and very annoying having to cast register values
>> every time they're passed in, instead of the compiler handling it for you.
>>
>> Is there a way to catch this with GCC? If not, I expect you to get broken
>> all the time on this by people who don't notice.
>
> Question to the clang people (Clangers?): what happens if the %0 above is
> rewritten as %x0 and the cast on val is dropped? I could stomach a change
> adding that, but it's still likely to regress without regular build testing.
>

I did a quick test with Clang, and indeed, it infers the type of
register from the size of the operand, but it also supports explicit
%x0 and %w0 casts.
So in this particular case, we could work around it in this way.

However, I think this uncovers a much more serious issue: while we
catch the problem here because msr simply does not support 32-bit
operands, most other instructions do, and we have no idea how the
Clang generated code deviates from what GCC produces. Or am I being
paranoid here?
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