Re: [PATCH v2 0/3] ARM: make use of UAL VFP mnemonics when possible

[Stefan Agner]

On 2020-04-19 16:12, Russell King - ARM Linux admin wrote:
> On Sun, Apr 19, 2020 at 02:35:48PM +0200, Stefan Agner wrote:
>> To build the kernel with Clang's integrated assembler the VFP code needs
>> to make use of the unified assembler language (UAL) VFP mnemonics.
>>
>> At first I tried to get rid of the co-processor instructions to access
>> the floating point unit along with the macros completely. However, due
>> to missing FPINST/FPINST2 argument support in older binutils versions we
>> have to keep them around. Once we drop support for binutils 2.24 and
>> older, the move to UAL VFP mnemonics will be straight forward with this
>> changes applied.
>>
>> Tested using Clang with integrated assembler as well as external
>> (binutils assembler), various gcc/binutils version down to 4.7/2.23.
>> Disassembled and compared the object files in arch/arm/vfp/ to make
>> sure this changes leads to the same code. Besides different inlining
>> behavior I was not able to spot a difference.
>>
>> In v2 the check for FPINST argument support is now made in Kconfig.
> 
> Given what I said in the other thread, Clang really _should_ allow
> the MCR/MRC et.al. instructions to access the VFP registers.  There
> is no reason to specifically block them.

I agree, and I am working on changing this.

There have been discussions about co-processor register access a while
back in the LLVM/Clang community [1]. Peter Smith pointed this out in
the ClangBuiltLinux issue tracker [2], which also has some more context.
I did submit a patch [3] to convert use of cp10/cp11 in ARMv7 contexts
to a warning. However it got stale, I'll have to revisit.

There is actually another case where this issue blocks Clang's
integrated assembler: In arch/arm/kernel/perf_event_v7.c, function
venum_read_pmresr mcr/mrc is used to access the performance monitor
registers for Qualcomm's Krait/Scorpion PMU, and in this case there is
no mnemonic available.

> 
> As we have seen with FPA, having that ability when iWMMXT comes along
> is very useful.  In any case:
> 
> 1. The ARM ARM (DDI0406) states that "These instructions are MRC and MCR
> instructions for coprocessors 10 and 11." in section A7.8.
> 
> 2. The ARM ARM (DDI0406) describes the MRC and MCR instructions as
> being able to access _any_ co-processor.

These are good arguments I can use in case my patch stirs up a
discussion, thanks for the hints!

> 
> So, Clang deciding that it's going to block access to coprocessor 10
> and 11 because some version of the architecture _also_ defines these
> as VFP instructions is really not on, and Clang needs to be fixed
> irrespective of these patches - and I want to know that *is* going to
> get fixed before I take these patches into the kernel.

I'll try. We'll see.

[1] https://bugs.llvm.org/show_bug.cgi?id=20025
[2] https://github.com/ClangBuiltLinux/linux/issues/306
[3] https://reviews.llvm.org/D59733

--
Stefan