linux-kernel - Re: [PATCH] LoongArch: Avoid using $r0/$r1 as "mask" for csrxchg

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Message-ID: <CAAhV-H5XyN2OZZqWm2_PCSj8Exs-FSNEL0nQ2AUXgwuTAJoQFw@mail.gmail.com>
Date: Fri, 23 May 2025 12:51:04 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Xi Ruoyao <xry111@...111.site>
Cc: Huacai Chen <chenhuacai@...ngson.cn>, loongarch@...ts.linux.dev, 
	Xuefeng Li <lixuefeng@...ngson.cn>, Guo Ren <guoren@...nel.org>, 
	Xuerui Wang <kernel@...0n.name>, Jiaxun Yang <jiaxun.yang@...goat.com>, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org, 
	WANG Rui <wangrui@...ngson.cn>
Subject: Re: [PATCH] LoongArch: Avoid using $r0/$r1 as "mask" for csrxchg

On Fri, May 23, 2025 at 12:38 PM Xi Ruoyao <xry111@...111.site> wrote:
>
> On Thu, 2025-05-22 at 20:50 +0800, Huacai Chen wrote:
> > When building kernel with LLVM there are occasionally such errors:
> >
> > In file included from ./include/linux/spinlock.h:59:
> > In file included from ./include/linux/irqflags.h:17:
> > arch/loongarch/include/asm/irqflags.h:38:3: error: must not be $r0 or $r1
> >    38 |                 "csrxchg %[val], %[mask], %[reg]\n\t"
> >       |                 ^
> > <inline asm>:1:16: note: instantiated into assembly here
> >     1 |         csrxchg $a1, $ra, 0
> >       |                       ^
> >
> > The "mask" of the csrxchg instruction should not be $r0 or $r1, but the
> > compiler cannot avoid generating such code currently.
>
> Maybe "to prevent the compiler from allocating $r0 or $r1, the 'q'
> constraint must be used but Clang < 22 does not support it.  So force to
> use t0 in order to avoid using $r0/$r1 while keeping the backward
> compatibility."
>
> And Link: https://github.com/llvm/llvm-project/pull/141037
OK, that's better.

Huacai

>
> > So force to use t0
> > in the inline asm, in order to avoid using $r0/$r1.
> >
> > Cc: stable@...r.kernel.org
> > Suggested-by: WANG Rui <wangrui@...ngson.cn>
> > Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> > ---
> >  arch/loongarch/include/asm/irqflags.h | 16 ++++++++++++----
> >  1 file changed, 12 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/loongarch/include/asm/irqflags.h b/arch/loongarch/include/asm/irqflags.h
> > index 319a8c616f1f..003172b8406b 100644
> > --- a/arch/loongarch/include/asm/irqflags.h
> > +++ b/arch/loongarch/include/asm/irqflags.h
> > @@ -14,40 +14,48 @@
> >  static inline void arch_local_irq_enable(void)
> >  {
> >       u32 flags = CSR_CRMD_IE;
> > +     register u32 mask asm("t0") = CSR_CRMD_IE;
> > +
> >       __asm__ __volatile__(
> >               "csrxchg %[val], %[mask], %[reg]\n\t"
> >               : [val] "+r" (flags)
> > -             : [mask] "r" (CSR_CRMD_IE), [reg] "i" (LOONGARCH_CSR_CRMD)
> > +             : [mask] "r" (mask), [reg] "i" (LOONGARCH_CSR_CRMD)
> >               : "memory");
> >  }
> >
> >  static inline void arch_local_irq_disable(void)
> >  {
> >       u32 flags = 0;
> > +     register u32 mask asm("t0") = CSR_CRMD_IE;
> > +
> >       __asm__ __volatile__(
> >               "csrxchg %[val], %[mask], %[reg]\n\t"
> >               : [val] "+r" (flags)
> > -             : [mask] "r" (CSR_CRMD_IE), [reg] "i" (LOONGARCH_CSR_CRMD)
> > +             : [mask] "r" (mask), [reg] "i" (LOONGARCH_CSR_CRMD)
> >               : "memory");
> >  }
> >
> >  static inline unsigned long arch_local_irq_save(void)
> >  {
> >       u32 flags = 0;
> > +     register u32 mask asm("t0") = CSR_CRMD_IE;
> > +
> >       __asm__ __volatile__(
> >               "csrxchg %[val], %[mask], %[reg]\n\t"
> >               : [val] "+r" (flags)
> > -             : [mask] "r" (CSR_CRMD_IE), [reg] "i" (LOONGARCH_CSR_CRMD)
> > +             : [mask] "r" (mask), [reg] "i" (LOONGARCH_CSR_CRMD)
> >               : "memory");
> >       return flags;
> >  }
> >
> >  static inline void arch_local_irq_restore(unsigned long flags)
> >  {
> > +     register u32 mask asm("t0") = CSR_CRMD_IE;
> > +
> >       __asm__ __volatile__(
> >               "csrxchg %[val], %[mask], %[reg]\n\t"
> >               : [val] "+r" (flags)
> > -             : [mask] "r" (CSR_CRMD_IE), [reg] "i" (LOONGARCH_CSR_CRMD)
> > +             : [mask] "r" (mask), [reg] "i" (LOONGARCH_CSR_CRMD)
> >               : "memory");
> >  }
> >
>
> --
> Xi Ruoyao <xry111@...111.site>
> School of Aerospace Science and Technology, Xidian University