linux-kernel - Re: [PATCH] ARM: use assembly mnemonics for VFP register access

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Message-ID: <0e4196b528284b07d088dec086f3378a@agner.ch>
Date:   Tue, 25 Feb 2020 21:00:32 +0100
From:   Stefan Agner <stefan@...er.ch>
To:     Robin Murphy <robin.murphy@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>, LKML <linux-kernel@...r.kernel.org>,
        Jian Cai <jiancai@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        Russell King <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: use assembly mnemonics for VFP register access

On 2020-02-25 20:45, Robin Murphy wrote:
> On 2020-02-25 7:33 pm, Ard Biesheuvel wrote:
>> On Tue, 25 Feb 2020 at 20:10, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>>>
>>> On Mon, Feb 24, 2020 at 9:22 PM Stefan Agner <stefan@...er.ch> wrote:
>>>>
>>>> Clang's integrated assembler does not allow to to use the mcr
>>>> instruction to access floating point co-processor registers:
>>>> arch/arm/vfp/vfpmodule.c:342:2: error: invalid operand for instruction
>>>>          fmxr(FPEXC, fpexc & ~(FPEXC_EX|FPEXC_DEX|FPEXC_FP2V|FPEXC_VV|FPEXC_TRAP_MASK));
>>>>          ^
>>>> arch/arm/vfp/vfpinstr.h:79:6: note: expanded from macro 'fmxr'
>>>>          asm("mcr p10, 7, %0, " vfpreg(_vfp_) ", cr0, 0 @ fmxr   " #_vfp_ ", %0" \
>>>>              ^
>>>> <inline asm>:1:6: note: instantiated into assembly here
>>>>          mcr p10, 7, r0, cr8, cr0, 0 @ fmxr      FPEXC, r0
>>>>              ^
>>>>
>>>> The GNU assembler supports the .fpu directive at least since 2.17 (when
>>>> documentation has been added). Since Linux requires binutils 2.21 it is
>>>> safe to use .fpu directive. Use the .fpu directive and mnemonics for VFP
>>>> register access.
>>>>
>>>> This allows to build vfpmodule.c with Clang and its integrated assembler.
>>>>
>>>> Link: https://github.com/ClangBuiltLinux/linux/issues/905
>>>> Signed-off-by: Stefan Agner <stefan@...er.ch>
>>>> ---
>>>>   arch/arm/vfp/vfpinstr.h | 12 ++++--------
>>>>   1 file changed, 4 insertions(+), 8 deletions(-)
>>>>
>>>> diff --git a/arch/arm/vfp/vfpinstr.h b/arch/arm/vfp/vfpinstr.h
>>>> index 38dc154e39ff..799ccf065406 100644
>>>> --- a/arch/arm/vfp/vfpinstr.h
>>>> +++ b/arch/arm/vfp/vfpinstr.h
>>>> @@ -62,21 +62,17 @@
>>>>   #define FPSCR_C (1 << 29)
>>>>   #define FPSCR_V        (1 << 28)
>>>>
>>>> -/*
>>>> - * Since we aren't building with -mfpu=vfp, we need to code
>>>> - * these instructions using their MRC/MCR equivalents.
>>>> - */
>>>> -#define vfpreg(_vfp_) #_vfp_
>>>> -
>>>>   #define fmrx(_vfp_) ({                 \
>>>>          u32 __v;                        \
>>>> -       asm("mrc p10, 7, %0, " vfpreg(_vfp_) ", cr0, 0 @ fmrx   %0, " #_vfp_    \
>>>> +       asm(".fpu       vfpv2\n"        \
>>>> +           "vmrs       %0, " #_vfp_    \
>>>>              : "=r" (__v) : : "cc");     \
>>>>          __v;                            \
>>>>    })
>>>>
>>>>   #define fmxr(_vfp_,_var_)              \
>>>> -       asm("mcr p10, 7, %0, " vfpreg(_vfp_) ", cr0, 0 @ fmxr   " #_vfp_ ", %0" \
>>>> +       asm(".fpu       vfpv2\n"        \
>>>> +           "vmsr       " #_vfp_ ", %0" \
>>>>             : : "r" (_var_) : "cc")
>>>>
>>>>   u32 vfp_single_cpdo(u32 inst, u32 fpscr);
>>>> --
>>>
>>> Hi Stefan,
>>> Thanks for the patch.  Reading through:
>>> - FMRX, FMXR, and FMSTAT:
>>> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0068b/Bcfbdihi.html
>>> - VMRS and VMSR:
>>> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204h/Bcfbdihi.html
>>>
>>> Should a macro called `fmrx` that had a comment about `fmrx` be using
>>> `vmrs` in place of `fmrx`?
>>>
>>> It looks like Clang treats them the same, but GCC keeps them separate:
>>> https://godbolt.org/z/YKmSAs
>>> Ah, this is only when streaming to assembly. Looks like they have the
>>> same encoding, and produce the same disassembly. (Godbolt emits
>>> assembly by default, and has the option to compile, then disassemble).
>>> If I take my case from godbolt above:
>>>
>>> ➜  /tmp arm-linux-gnueabihf-gcc -O2 -c x.c
>>> ➜  /tmp llvm-objdump -dr x.o
>>>
>>> x.o: file format elf32-arm-little
>>>
>>>
>>> Disassembly of section .text:
>>>
>>> 00000000 bar:
>>>         0: f1 ee 10 0a                  vmrs r0, fpscr
>>>         4: 70 47                        bx lr
>>>         6: 00 bf                        nop
>>>
>>> 00000008 baz:
>>>         8: f1 ee 10 0a                  vmrs r0, fpscr
>>>         c: 70 47                        bx lr
>>>         e: 00 bf                        nop
>>>
>>> So indeed a similar encoding exists for the two different assembler
>>> instructions.
>>
>> Does that hold for ARM (A32) instructions as well?
> 
> It should do - they're all the same thing underneath. The UAL syntax
> just renamed all the legacy VFP mnemonics from Fxxx to Vxxx form,
> apart from a couple of things that were already deprecated. GAS still
> accepts both regardless of ".syntax unified", and as a result GCC
> never saw a reason to stop emitting the old mnemonics.
> 

Yes this is really only a mnemonic change when unified assembler
language (UAL) got introduce, the ARM ARM has a list of mnemonic changes
in the appendix.

Just do make sure I also did compare the disassembled object file of
vfpmodule.c before and after this change.

I guess we could (should?) also change the macro name, but I guess that
should be a separate commit anyway.

--
Stefan