linux-kernel - Re: [PATCH v1 1/8] tools/nolibc: add support for powerpc

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Date:   Sun, 23 Jul 2023 09:32:37 +0200
From:   Thomas Weißschuh <thomas@...ch.de>
To:     Zhangjin Wu <falcon@...ylab.org>
Cc:     w@....eu, arnd@...db.de, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v1 1/8] tools/nolibc: add support for powerpc

On 2023-07-19 05:10:48+0800, Zhangjin Wu wrote:
> Both syscall declarations and _start code definition are added for
> powerpc to nolibc.
> 
> Like mips, powerpc uses a register (exactly, the summary overflow bit)
> to record the error occurred, and uses another register to return the
> value [1]. So, the return value of every syscall declaration must be
> normalized to easier the __sysret helper, return -value when there is an
> error, otheriwse, return value directly.
> 
> Glibc and musl use different methods to check the summary overflow bit,
> glibc (sysdeps/unix/sysv/linux/powerpc/sysdep.h) saves the cr register
> to r0 at first, and then check the summary overflow bit in cr0:
> 
>     mfcr r0
>     r0 & (1 << 28) ? -r3 : r3
> 
>     -->
> 
>     10003c14:       7c 00 00 26     mfcr    r0
>     10003c18:       74 09 10 00     andis.  r9,r0,4096
>     10003c1c:       41 82 00 08     beq     0x10003c24
>     10003c20:       7c 63 00 d0     neg     r3,r3
> 
> Musl (arch/powerpc/syscall_arch.h) directly checks the summary overflow
> bit with the 'bns' instruction:
> 
>     /* no summary overflow bit means no error, return value directly */
>     bns+ 1f
>     /* otherwise, return negated value */
>     neg r3, r3
>     1:
> 
>     -->
> 
>     10000418:       40 a3 00 08     bns     0x10000420
>     1000041c:       7c 63 00 d0     neg     r3,r3
> 
> The later one is smaller, here applies it.
> 
> arch/powerpc/include/asm/vdso/gettimeofday.h file uses the smaller
> method for do_syscall_2() too.
> 
> [1]: https://man7.org/linux/man-pages/man2/syscall.2.html
> 
> Signed-off-by: Zhangjin Wu <falcon@...ylab.org>
> ---
>  tools/include/nolibc/arch-powerpc.h | 156 ++++++++++++++++++++++++++++

This also should be added to nolibc/arch.h.

>  1 file changed, 156 insertions(+)
>  create mode 100644 tools/include/nolibc/arch-powerpc.h
> 
> diff --git a/tools/include/nolibc/arch-powerpc.h b/tools/include/nolibc/arch-powerpc.h
> new file mode 100644
> index 000000000000..100ec0f412dc
> --- /dev/null
> +++ b/tools/include/nolibc/arch-powerpc.h
> @@ -0,0 +1,156 @@
> +/* SPDX-License-Identifier: LGPL-2.1 OR MIT */
> +/*
> + * PowerPC specific definitions for NOLIBC
> + * Copyright (C) 2023 Zhangjin Wu <falcon@...ylab.org>

If it is taken from musl, shouldn't there also be a musl copyright?

> [..]