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

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Message-ID: <20230725-skating-agent-b092f2257ca1@spud>
Date:   Tue, 25 Jul 2023 19:57:54 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Mingzheng Xing <xingmingzheng@...as.ac.cn>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, Bin Meng <bmeng@...ylab.org>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] riscv: Handle zicsr/zifencei issue between gcc and
 binutils

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.

> > 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@...r.kernel.org too.

Cheers,
Conor.

> > 
> > Signed-off-by: Mingzheng Xing <xingmingzheng@...as.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
> > 


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