linux-kernel - Re: [RFC PATCH v2 3/8] tools/nolibc: i386: Implement syscall with 6 arguments

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Date:   Tue, 22 Mar 2022 19:02:53 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     David Laight <David.Laight@...LAB.COM>, Willy Tarreau <w@....eu>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>,
        Nugraha <richiisei@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: Re: [RFC PATCH v2 3/8] tools/nolibc: i386: Implement syscall with 6
 arguments

On 3/22/22 6:39 PM, David Laight wrote:
>> +	__asm__ volatile (
>> +		"pushl	%[arg6]\n\t"
>> +		"pushl	%%ebp\n\t"
>> +		"movl	4(%%esp), %%ebp\n\t"
>> +		"int	$0x80\n\t"
>> +		"popl	%%ebp\n\t"
>> +		"addl	$4,%%esp\n\t"
>> +		: "=a"(eax)
>> +		: "a"(eax), "b"(ebx), "c"(ecx), "d"(edx), "S"(esi), "D"(edi),
> 
> Does having "=a" for an output constraint and "a" for an input
> constraint actually DTRT?
> There is a special syntax for tying input and output to
> the same register.
> Or you could use "+a"(nr_rval) and 'return nr_rval'.

Well, I agree with your previous email. Now since we no longer use a #pragma
optimize with -fomit-frame-pointer, the function is not needed. I propose the
following macro (this is not so much different with other my_syscall macro),
expect the 6th argument can be in reg or mem.

The "rm" constraint here gives the opportunity for the compiler to use %ebp
instead of memory if -fomit-frame-pointer is turned on.

#define my_syscall6(num, arg1, arg2, arg3, arg4, arg5, arg6) \
({                                                         \
     long _ret;                                             \
     register long _num asm("eax") = (num);                 \
     register long _arg1 asm("ebx") = (long)(arg1);         \
     register long _arg2 asm("ecx") = (long)(arg2);         \
     register long _arg3 asm("edx") = (long)(arg3);         \
     register long _arg4 asm("esi") = (long)(arg4);         \
     register long _arg5 asm("edi") = (long)(arg5);         \
     long _arg6 = (long)(arg6); /* Might be in memory */    \
                                                            \
     asm volatile (                                         \
         "pushl  %[_arg6]\n\t"                              \
         "pushl  %%ebp\n\t"                                 \
         "movl   4(%%esp), %%ebp\n\t"                       \
         "int    $0x80\n\t"                                 \
         "popl   %%ebp\n\t"                                 \
         "addl   $4,%%esp\n\t"                              \
         : "=a"(_ret)                                       \
         : "r"(_num), "r"(_arg1), "r"(_arg2), "r"(_arg3),   \
           "r"(_arg4),"r"(_arg5), [_arg6]"rm"(_arg6)        \
         : "memory", "cc"                                   \
     );                                                     \
     _ret;                                                  \
})

What do you think?

-- 
Ammar Faizi