linux-kernel - Re: [PATCH] tools/nolibc: add support for N64 and N32 ABIs

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Message-ID: <alpine.DEB.2.21.2502161523290.65342@angie.orcam.me.uk>
Date: Sun, 16 Feb 2025 15:41:55 +0000 (GMT)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: Thomas Weißschuh <linux@...ssschuh.net>
cc: Willy Tarreau <w@....eu>, Shuah Khan <shuah@...nel.org>, 
    Thomas Bogendoerfer <tsbogend@...ha.franken.de>, 
    linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
    linux-mips@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] tools/nolibc: add support for N64 and N32 ABIs

On Wed, 12 Feb 2025, Thomas Weißschuh wrote:

> diff --git a/tools/include/nolibc/arch-mips.h b/tools/include/nolibc/arch-mips.h
> index 753a8ed2cf695f0b5eac4b5e4d317fdb383ebf93..638520a3427a985fdbd5f5a49b55853bbadeee75 100644
> --- a/tools/include/nolibc/arch-mips.h
> +++ b/tools/include/nolibc/arch-mips.h
> @@ -190,13 +257,33 @@ void __attribute__((weak, noreturn)) __nolibc_entrypoint __no_stack_protector __
>  		"1:\n"
>  		".cpload $ra\n"
>  		"move  $a0, $sp\n"       /* save stack pointer to $a0, as arg1 of _start_c */
> +
> +#if defined(_ABIO32)
>  		"addiu $sp, $sp, -4\n"   /* space for .cprestore to store $gp              */
>  		".cprestore 0\n"
>  		"li    $t0, -8\n"
>  		"and   $sp, $sp, $t0\n"  /* $sp must be 8-byte aligned                     */
>  		"addiu $sp, $sp, -16\n"  /* the callee expects to save a0..a3 there        */
> -		"lui $t9, %hi(_start_c)\n" /* ABI requires current function address in $t9 */
> +#else
> +		"daddiu $sp, $sp, -8\n"  /* space for .cprestore to store $gp              */
> +		".cpsetup $ra, 0, 1b\n"
> +		"li    $t0, -16\n"
> +		"and   $sp, $sp, $t0\n"  /* $sp must be 16-byte aligned                    */
> +#endif

 Why is this code breaking stack alignment just to have to fix it up two 
instructions down the line?  Or is it that the incoming $sp is not aligned 
in the first place (in which case we're having a deeper problem).

> +
> +		/* ABI requires current function address in $t9 */
> +#if defined(_ABIO32) || defined(_ABIN32)
> +		"lui $t9, %hi(_start_c)\n"
>  		"ori $t9, %lo(_start_c)\n"
> +#else
> +		"lui  $t9, %highest(_start_c)\n"
> +		"ori  $t9, %higher(_start_c)\n"
> +		"dsll $t9, 0x10\n"
> +		"ori  $t9, %hi(_start_c)\n"
> +		"dsll $t9, 0x10\n"
> +		"ori  $t9, %lo(_start_c)\n"

 This could be optimised using a temporary (e.g. $at, but I guess any will 
do as I gather we don't have any ABI abnormalities here).

> +#endif
> +
>  		"jalr $t9\n"             /* transfer to c runtime
> */
>  		" nop\n"                 /* delayed slot

 On an unrelated matter JALR above ought to be JAL (or otherwise there's 
no point in using the .cprestore pseudo-op).  And I fail to see why this 
code has to be "noreorder" (except for the .cpload piece, of course), it's 
just asking for troubles.

  Maciej