Re: [PATCH] riscv: Handle zicsr/zifencei issue between gcc and binutils

[Mingzheng Xing <xingmingzheng@iscas.ac.cn>]

On 7/26/23 02:57, Conor Dooley wrote:
> On Tue, Jul 25, 2023 at 10:23:44AM -0700, Nathan Chancellor wrote:
>> Hi Mingzheng,
>>
>> Thanks for the patch!
>>
>> On Wed, Jul 26, 2023 at 01:04:05AM +0800, Mingzheng Xing wrote:
>>> When compiling the kernel with the toolchain composed of GCC >= 12.1.0 and
>>> binutils < 2.38, default ISA spec used when building binutils and GCC, the
>>> following build failure will appear because the
>>> CONFIG_TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI is not turned on.
>>> (i.e, gcc-12.1.0 and binutils-2.36, or gcc-12.3.0 and binutils-2.37, use
>>> default ISA spec.)
>>>
>>>    CC      arch/riscv/kernel/vdso/vgettimeofday.o
>>>    <<BUILDDIR>>/arch/riscv/include/asm/vdso/gettimeofday.h: Assembler messages:
>>>    <<BUILDDIR>>/arch/riscv/include/asm/vdso/gettimeofday.h:79: Error: unrecognized opcode `csrr a5,0xc01'
>> The gift that keeps on giving :/
>>
>>> Binutils has updated the default ISA spec version, and the community has
>>> responded well to this[1][2][3], but it appears that this is not over yet.
>>>
>>> We also need to consider the situation of binutils < 2.38 but
>>> GCC >= 12.1.0, since the combination between different versions of GCC and
>>> binutils is not unique, which is to some extent flexible. GCC release
>>> 12.1.0 updated the default ISA spec version in GCC commit[4].
>> I suspect this combination is not too common because binutils 2.38 came
>> out before GCC 12.1.0 but as you note, it is obviously possible. What
>> toolchain has this combination in the wild, which would be helpful for
>> documentation purposes?
> Yeah, that'd be great to know, at least the other niche stuff that we
> are working around had a clear use-case (testing LLVM in debian containers)
> whereas there's no clear user for this.
> That's doubly interesting, as this patch seems to break things for binutils
> < 2.35, and if we have to make a trade-off between those too, then it'd
> be good to be able to weigh up the options.
> Do we perhaps need the misa-spec workaround instead for this case?
> Haven't tested that though, trying to dig myself out of email backlog.
Well, what I encountered use-case was a temporary thing caused by the
inconsistent pace of distro package upgrades, but it really happened and
took quite a bit of time to explore why. There are sites like [1] that count
the "Successful Builds" between different GCC and binutils releases,
(Though they don't seem to be updated much...), but it seems to indicate
that all kinds of available pairings are possible.

Before replying to the e-mail, I tested some "extreme" cases and things
seemed to be a little clearer.

1. binutils-2.38 and GCC-12.1.0 each changed default ISA spec version,
     updating from 2.2 to 20191213.

2. binutils>=2.38 or GCC>=12.1.0
     when meet any of these it is recommended to turn on
     TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI, the good outweighs
     the bad. (My personal understanding.)

3. (Extreme case) binutils>=2.38 AND GCC<11.1.0
     GCC-11.1.0 starts to support zicsr and zifencei extension for 
-march[2].
     In this case just turn on TOOLCHAIN_NEEDS_OLD_ISA_SPEC.
     (Otherwise compiling the kernel will report an error whether
     TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI is turned on or not.)

4. (Extreme case) GCC>=12.1.0, binutils<2.36
     I tested GCC-12.1.0 + binutils-2.36 and it compiles the kernel 
fine(after
     hitting this patch). Also tested GCC-12.1.0 + binutils2.35 and
     GCC-12.1.0 + binutils-2.34.0, but both pairings gave errors and failed
     to produce a usable toolchain (Default ISA versions used). It seems 
safe
     to assume that GCC-12.1.0 + binutils<2.35 is almost non-existent.

I'm no expert on toolchains, so thanks for correcting me if I'm wrong 
somewhere...

[1] https://wiki.osdev.org/Cross-Compiler_Successful_Builds
[2] 
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=b03be74bad08c382da47e048007a78fa3fb4ef49
>>> For more information, please refer to:
>>>
>>> commit 6df2a016c0c8 ("riscv: fix build with binutils 2.38")
>>> commit e89c2e815e76 ("riscv: Handle zicsr/zifencei issues between clang and binutils")
>>>
>>> [1]:https://groups.google.com/a/groups.riscv.org/g/sw-dev/c/aE1ZeHHCYf4
>>> [2]:https://lore.kernel.org/all/20230308220842.1231003-1-conor@kernel.org
>>> [3]:https://lore.kernel.org/all/20230223220546.52879-1-conor@kernel.org
>>> [4]:https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=98416dbb0a62579d4a7a4a76bab51b5b52fec2cd
> btw, please make these regular Link: tags (with a [N] at EOL) and drop
> the space between them and the sign off. Also, this probably needs to be
> CC:stable@vger.kernel.org  too.
OK, I'll fix it.

Thanks,
Mingzheng.
> Cheers,
> Conor.
>
>>> Signed-off-by: Mingzheng Xing<xingmingzheng@iscas.ac.cn>
>>> ---
>>>   arch/riscv/Kconfig | 6 +++++-
>>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>>> index 4c07b9189c86..b49cea30f6cc 100644
>>> --- a/arch/riscv/Kconfig
>>> +++ b/arch/riscv/Kconfig
>>> @@ -570,11 +570,15 @@ config TOOLCHAIN_HAS_ZIHINTPAUSE
>>>   config TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI
>>>   	def_bool y
>>>   	#https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=aed44286efa8ae8717a77d94b51ac3614e2ca6dc
>>> -	depends on AS_IS_GNU && AS_VERSION >= 23800
>>> +	#https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=98416dbb0a62579d4a7a4a76bab51b5b52fec2cd
>>> +	depends on CC_IS_GCC && GCC_VERSION >= 120100 || \
>>> +		   AS_IS_GNU && AS_VERSION >= 23800
>> GCC_VERSION will be 0 for clang, so you don't need the CC_IS_GCC check.
>> With that change, this should be able to stay on one line:
>>
>>      depends on GCC_VERSION >= 120100 || (AS_IS_GNU && AS_VERSION >= 23800)
>>
>>>   	help
>>>   	  Newer binutils versions default to ISA spec version 20191213 which
>>>   	  moves some instructions from the I extension to the Zicsr and Zifencei
>>>   	  extensions.
>>> +	  Similarly, GCC release 12.1.0 has changed the default ISA spec version to
>>> +	  20191213, so the above situation requires this option to be enabled.
>>>   
>>>   config TOOLCHAIN_NEEDS_OLD_ISA_SPEC
>>>   	def_bool y
>>> -- 
>>> 2.34.1
>>>