Re: [PATCH] mips: Do not include hi and lo in clobber list for R6

[Alexander Lobakin <alobakin@pm.me>]

Hi Romain,

From: Romain Naour <romain.naour@gmail.com>
Date: Sat, 25 Jul 2020 15:25:33 +0200

> From [1]
> "GCC 10 (PR 91233) won't silently allow registers that are not architecturally
> available to be present in the clobber list anymore, resulting in build failure
> for mips*r6 targets in form of:
> ...
> .../sysdep.h:146:2: error: the register ‘lo’ cannot be clobbered in ‘asm’ for the current target
>   146 |  __asm__ volatile (      \
>       |  ^~~~~~~
> 
> This is because base R6 ISA doesn't define hi and lo registers w/o DSP extension.
> This patch provides the alternative clobber list for r6 targets that won't include
> those registers."
> 
> Since kernel 5.4 and mips support for generic vDSO [2], the kernel fail to build
> for mips r6 cpus with gcc 10 for the same reason as glibc.
> 
> [1] https://sourceware.org/git/?p=glibc.git;a=commit;h=020b2a97bb15f807c0482f0faee2184ed05bcad8
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=24640f233b466051ad3a5d2786d2951e43026c9d
> 
> Signed-off-by: Romain Naour <romain.naour@gmail.com>
> ---
>  arch/mips/include/asm/vdso/gettimeofday.h | 45 +++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
> 
> diff --git a/arch/mips/include/asm/vdso/gettimeofday.h b/arch/mips/include/asm/vdso/gettimeofday.h
> index c63ddcaea54c..6192d94928b9 100644
> --- a/arch/mips/include/asm/vdso/gettimeofday.h
> +++ b/arch/mips/include/asm/vdso/gettimeofday.h
> @@ -30,12 +30,21 @@ static __always_inline long gettimeofday_fallback(
>  	register long nr asm("v0") = __NR_gettimeofday;
>  	register long error asm("a3");
>  
> +#if __mips_isa_rev >= 6

You should use MIPS_ISA_REV macro from arch/mips/include/asm/isa-rev.h
instead of compiler definitions.
The main reason is that __mips_isa_rev is not defined by GCC for
pre-MIPS32 processors, so you'll get a compiler warning like:

"Warning: macro '__mips_isa_rev' is not defined, evaluates to 0"

MIPS_ISA_REV handles this case and is always defined regardless of
the actual ISA family.

> +	asm volatile(
> +	"       syscall\n"
> +	: "=r" (ret), "=r" (error)
> +	: "r" (tv), "r" (tz), "r" (nr)
> +	: "$1", "$3", "$8", "$9", "$10", "$11", "$12", "$13",
> +	  "$14", "$15", "$24", "$25", "memory");
> +#else
>  	asm volatile(
>  	"       syscall\n"
>  	: "=r" (ret), "=r" (error)
>  	: "r" (tv), "r" (tz), "r" (nr)
>  	: "$1", "$3", "$8", "$9", "$10", "$11", "$12", "$13",
>  	  "$14", "$15", "$24", "$25", "hi", "lo", "memory");
> +#endif
>  
>  	return error ? -ret : ret;
>  }
> @@ -54,12 +63,21 @@ static __always_inline long clock_gettime_fallback(
>  #endif
>  	register long error asm("a3");
>  
> +#if __mips_isa_rev >= 6
> +	asm volatile(
> +	"       syscall\n"
> +	: "=r" (ret), "=r" (error)
> +	: "r" (clkid), "r" (ts), "r" (nr)
> +	: "$1", "$3", "$8", "$9", "$10", "$11", "$12", "$13",
> +	  "$14", "$15", "$24", "$25", "memory");
> +#else
>  	asm volatile(
>  	"       syscall\n"
>  	: "=r" (ret), "=r" (error)
>  	: "r" (clkid), "r" (ts), "r" (nr)
>  	: "$1", "$3", "$8", "$9", "$10", "$11", "$12", "$13",
>  	  "$14", "$15", "$24", "$25", "hi", "lo", "memory");
> +#endif
>  
>  	return error ? -ret : ret;
>  }
> @@ -78,12 +96,21 @@ static __always_inline int clock_getres_fallback(
>  #endif
>  	register long error asm("a3");
>  
> +#if __mips_isa_rev >= 6
> +	asm volatile(
> +	"       syscall\n"
> +	: "=r" (ret), "=r" (error)
> +	: "r" (clkid), "r" (ts), "r" (nr)
> +	: "$1", "$3", "$8", "$9", "$10", "$11", "$12", "$13",
> +	  "$14", "$15", "$24", "$25", "memory");
> +#else
>  	asm volatile(
>  	"       syscall\n"
>  	: "=r" (ret), "=r" (error)
>  	: "r" (clkid), "r" (ts), "r" (nr)
>  	: "$1", "$3", "$8", "$9", "$10", "$11", "$12", "$13",
>  	  "$14", "$15", "$24", "$25", "hi", "lo", "memory");
> +#endif
>  
>  	return error ? -ret : ret;
>  }
> @@ -100,12 +127,21 @@ static __always_inline long clock_gettime32_fallback(
>  	register long nr asm("v0") = __NR_clock_gettime;
>  	register long error asm("a3");
>  
> +#if __mips_isa_rev >= 6
> +	asm volatile(
> +	"       syscall\n"
> +	: "=r" (ret), "=r" (error)
> +	: "r" (clkid), "r" (ts), "r" (nr)
> +	: "$1", "$3", "$8", "$9", "$10", "$11", "$12", "$13",
> +	  "$14", "$15", "$24", "$25", "memory");
> +#else
>  	asm volatile(
>  	"       syscall\n"
>  	: "=r" (ret), "=r" (error)
>  	: "r" (clkid), "r" (ts), "r" (nr)
>  	: "$1", "$3", "$8", "$9", "$10", "$11", "$12", "$13",
>  	  "$14", "$15", "$24", "$25", "hi", "lo", "memory");
> +#endif
>  
>  	return error ? -ret : ret;
>  }
> @@ -120,12 +156,21 @@ static __always_inline int clock_getres32_fallback(
>  	register long nr asm("v0") = __NR_clock_getres;
>  	register long error asm("a3");
>  
> +#if __mips_isa_rev >= 6
> +	asm volatile(
> +	"       syscall\n"
> +	: "=r" (ret), "=r" (error)
> +	: "r" (clkid), "r" (ts), "r" (nr)
> +	: "$1", "$3", "$8", "$9", "$10", "$11", "$12", "$13",
> +	  "$14", "$15", "$24", "$25", "memory");
> +#else
>  	asm volatile(
>  	"       syscall\n"
>  	: "=r" (ret), "=r" (error)
>  	: "r" (clkid), "r" (ts), "r" (nr)
>  	: "$1", "$3", "$8", "$9", "$10", "$11", "$12", "$13",
>  	  "$14", "$15", "$24", "$25", "hi", "lo", "memory");
> +#endif
>  
>  	return error ? -ret : ret;
>  }
> -- 
> 2.25.4