linux-kernel - Re: [PATCH] LoongArch: vDSO: correctly use asm parameters in syscall wrappers

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Message-ID: <5a5329feaab84acb91bbb4f48ea548b3fb4eab0f.camel@xry111.site>
Date: Wed, 04 Jun 2025 22:30:55 +0800
From: Xi Ruoyao <xry111@...111.site>
To: Huacai Chen <chenhuacai@...nel.org>, Thomas Weißschuh
	 <thomas.weissschuh@...utronix.de>
Cc: WANG Xuerui <kernel@...0n.name>, Theodore Ts'o <tytso@....edu>, "Jason
 A. Donenfeld" <Jason@...c4.com>, Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers	 <nick.desaulniers+lkml@...il.com>, Bill Wendling
 <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, Jiaxun Yang
 <jiaxun.yang@...goat.com>, 	loongarch@...ts.linux.dev,
 linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, 	stable@...r.kernel.org
Subject: Re: [PATCH] LoongArch: vDSO: correctly use asm parameters in
 syscall wrappers

On Wed, 2025-06-04 at 22:05 +0800, Huacai Chen wrote:
> On Tue, Jun 3, 2025 at 7:49 PM Thomas Weißschuh
> <thomas.weissschuh@...utronix.de> wrote:
> > 
> > The syscall wrappers use the "a0" register for two different register
> > variables, both the first argument and the return value. The "ret"
> > variable is used as both input and output while the argument register is
> > only used as input. Clang treats the conflicting input parameters as
> > undefined behaviour and optimizes away the argument assignment.
> > 
> > The code seems to work by chance for the most part today but that may
> > change in the future. Specifically clock_gettime_fallback() fails with
> > clockids from 16 to 23, as implemented by the upcoming auxiliary clocks.
> > 
> > Switch the "ret" register variable to a pure output, similar to the other
> > architectures' vDSO code. This works in both clang and GCC.
> Hmmm, at first the constraint is "=r", during the progress of
> upstream, Xuerui suggested me to use "+r" instead [1].
> [1]  https://lore.kernel.org/linux-arch/5b14144a-9725-41db-7179-c059c41814cf@xen0n.name/

Based on the example at
https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html:

   To force an operand into a register, create a local variable and specify
   the register name after the variable’s declaration. Then use the local
   variable for the asm operand and specify any constraint letter that
   matches the register:
   
   register int *p1 asm ("r0") = …;
   register int *p2 asm ("r1") = …;
   register int *result asm ("r0");
   asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2));
   
I think this should actually be written

 	asm volatile(
 	"      syscall 0\n"
	: "=r" (ret)
 	: "r" (nr), "0" (buffer), "r" (len), "r" (flags)
 	: "$t0", "$t1", "$t2", "$t3", "$t4", "$t5", "$t6", "$t7",
"$t8",
 	  "memory");

i.e. "=" should be used for the output operand 0, and "0" should be used
for the input operand 2 (buffer) to emphasis the same register as
operand 0 is used.

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University