From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on gnuweeb.org X-Spam-Level: X-Spam-Status: No, score=-0.7 required=5.0 tests=PDS_BAD_THREAD_QP_64, RCVD_IN_DNSWL_LOW,RCVD_IN_MSPIKE_H5,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE, SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.86.151]) by gnuweeb.org (Postfix) with ESMTPS id E79CD7E337 for ; Tue, 22 Mar 2022 10:57:04 +0000 (UTC) Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-9-Lon-t17yP1qoUoOghi9yew-1; Tue, 22 Mar 2022 10:57:01 +0000 X-MC-Unique: Lon-t17yP1qoUoOghi9yew-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) by AcuMS.aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) with Microsoft SMTP Server (TLS) id 15.0.1497.32; Tue, 22 Mar 2022 10:57:02 +0000 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.033; Tue, 22 Mar 2022 10:57:01 +0000 From: David Laight To: 'Ammar Faizi' , Willy Tarreau CC: "Paul E. McKenney" , Alviro Iskandar Setiawan , Nugraha , "Linux Kernel Mailing List" , GNU/Weeb Mailing List , "x86@kernel.org" , "llvm@lists.linux.dev" Subject: RE: [RFC PATCH v2 3/8] tools/nolibc: i386: Implement syscall with 6 arguments Thread-Topic: [RFC PATCH v2 3/8] tools/nolibc: i386: Implement syscall with 6 arguments Thread-Index: AQHYPdalMvftIu2CIkSH1LImoy2kmazLOrmQ Date: Tue, 22 Mar 2022 10:57:01 +0000 Message-ID: <35ec31089f724a74a584f946deac6248@AcuMS.aculab.com> References: <20220322102115.186179-1-ammarfaizi2@gnuweeb.org> <20220322102115.186179-4-ammarfaizi2@gnuweeb.org> In-Reply-To: <20220322102115.186179-4-ammarfaizi2@gnuweeb.org> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable List-Id: From: Ammar Faizi > Sent: 22 March 2022 10:21 >=20 > On i386, the 6th argument of syscall goes in %ebp. However, both Clang > and GCC cannot use %ebp in the clobber list and in the "r" constraint > without using -fomit-frame-pointer. To make it always available for > any kind of compilation, the below workaround is implemented. >=20 > For clang (the Assembly statement can't clobber %ebp): > 1) Push the 6-th argument. > 2) Push %ebp. > 3) Load the 6-th argument from 4(%esp) to %ebp. > 4) Do the syscall (int $0x80). > 5) Pop %ebp (restore the old value of %ebp). > 6) Add %esp by 4 (undo the stack pointer). >=20 > For GCC, fortunately it has a #pragma that can force a specific function > to be compiled with -fomit-frame-pointer, so it can use "r"(var) where > var is a variable bound to %ebp. You need to use the 'clang' pattern for gcc. #pragma optimise is fundamentally broken. What actually happens here is the 'inline' gets lost (because of the implied -O0) and you get far worse code than you might expect. Since you need the 'clang' version, use it all the time. =09David >=20 > Cc: x86@kernel.org > Cc: llvm@lists.linux.dev > Link: https://lore.kernel.org/lkml/2e335ac54db44f1d8496583d97f9dab0@AcuMS= .aculab.com > Suggested-by: David Laight > Signed-off-by: Ammar Faizi > --- >=20 > @@ Changelog: >=20 > Link RFC v1: https://lore.kernel.org/llvm/20220320093750.159991-4-amma= rfaizi2@gnuweeb.org > RFC v1 -> RFC v2: > - Fix %ebp saving method. Don't use redzone, i386 doesn't have a redz= one > (comment from David and Alviro). > --- > tools/include/nolibc/arch-i386.h | 66 ++++++++++++++++++++++++++++++++ > 1 file changed, 66 insertions(+) >=20 > diff --git a/tools/include/nolibc/arch-i386.h b/tools/include/nolibc/arch= -i386.h > index 125a691fc631..9f4dc36e6ac2 100644 > --- a/tools/include/nolibc/arch-i386.h > +++ b/tools/include/nolibc/arch-i386.h > @@ -167,6 +167,72 @@ struct sys_stat_struct { > =09_ret; = \ > }) >=20 > + > +/* > + * Both Clang and GCC cannot use %ebp in the clobber list and in the "r" > + * constraint without using -fomit-frame-pointer. To make it always > + * available for any kind of compilation, the below workaround is > + * implemented. > + * > + * For clang (the Assembly statement can't clobber %ebp): > + * 1) Push the 6-th argument. > + * 2) Push %ebp. > + * 3) Load the 6-th argument from 4(%esp) to %ebp. > + * 4) Do the syscall (int $0x80). > + * 5) Pop %ebp (restore the old value of %ebp). > + * 6) Add %esp by 4 (undo the stack pointer). > + * > + * For GCC, fortunately it has a #pragma that can force a specific funct= ion > + * to be compiled with -fomit-frame-pointer, so it can use "r"(var) wher= e > + * var is a variable bound to %ebp. > + * > + */ > +#if defined(__clang__) > +static inline long ____do_syscall6(long eax, long ebx, long ecx, long ed= x, > +=09=09=09=09 long esi, long edi, long ebp) > +{ > +=09__asm__ volatile ( > +=09=09"pushl=09%[arg6]\n\t" > +=09=09"pushl=09%%ebp\n\t" > +=09=09"movl=094(%%esp), %%ebp\n\t" > +=09=09"int=09$0x80\n\t" > +=09=09"popl=09%%ebp\n\t" > +=09=09"addl=09$4,%%esp\n\t" > +=09=09: "=3Da"(eax) > +=09=09: "a"(eax), "b"(ebx), "c"(ecx), "d"(edx), "S"(esi), "D"(edi), > +=09=09 [arg6]"m"(ebp) > +=09=09: "memory", "cc" > +=09); > +=09return eax; > +} > + > +#else /* #if defined(__clang__) */ > +#pragma GCC push_options > +#pragma GCC optimize "-fomit-frame-pointer" > +static long ____do_syscall6(long eax, long ebx, long ecx, long edx, long= esi, > +=09=09=09 long edi, long ebp) > +{ > +=09register long __ebp __asm__("ebp") =3D ebp; > +=09__asm__ volatile ( > +=09=09"int=09$0x80" > +=09=09: "=3Da"(eax) > +=09=09: "a"(eax), "b"(ebx), "c"(ecx), "d"(edx), "S"(esi), "D"(edi), > +=09=09 "r"(__ebp) > +=09=09: "memory", "cc" > +=09); > +=09return eax; > +} > +#pragma GCC pop_options > +#endif /* #if defined(__clang__) */ > + > +#define my_syscall6(num, arg1, arg2, arg3, arg4, arg5, arg6) ( \ > +=09____do_syscall6((long)(num), (long)(arg1), \ > +=09=09=09(long)(arg2), (long)(arg3), \ > +=09=09=09(long)(arg4), (long)(arg5), \ > +=09=09=09(long)(arg6)) \ > +) > + > + > /* startup code */ > /* > * i386 System V ABI mandates: > -- > Ammar Faizi - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)