From: Ammar Faizi ammar.faizi@students.amikom.ac.id
[ Upstream commit bf91666959eeac44fb686e9359e37830944beef2 ]
Linux x86-64 syscall only clobbers rax, rcx and r11 (and "memory").
- rax for the return value. - rcx to save the return address. - r11 to save the rflags.
Other registers are preserved.
Having r8, r9 and r10 in the syscall clobber list is harmless, but this results in a missed-optimization.
As the syscall doesn't clobber r8-r10, GCC should be allowed to reuse their value after the syscall returns to userspace. But since they are in the clobber list, GCC will always miss this opportunity.
Remove them from the x86-64 syscall clobber list to help GCC generate better code and fix the comment.
See also the x86-64 ABI, section A.2 AMD64 Linux Kernel Conventions, A.2.1 Calling Conventions [1].
Extra note: Some people may think it does not really give a benefit to remove r8, r9 and r10 from the syscall clobber list because the impression of syscall is a C function call, and function call always clobbers those 3.
However, that is not the case for nolibc.h, because we have a potential to inline the "syscall" instruction (which its opcode is "0f 05") to the user functions.
All syscalls in the nolibc.h are written as a static function with inline ASM and are likely always inline if we use optimization flag, so this is a profit not to have r8, r9 and r10 in the clobber list.
Here is the example where this matters.
Consider the following C code: ``` #include "tools/include/nolibc/nolibc.h" #define read_abc(a, b, c) __asm__ volatile("nop"::"r"(a),"r"(b),"r"(c))
int main(void) { int a = 0xaa; int b = 0xbb; int c = 0xcc;
read_abc(a, b, c); write(1, "test\n", 5); read_abc(a, b, c);
return 0; } ```
Compile with: gcc -Os test.c -o test -nostdlib
With r8, r9, r10 in the clobber list, GCC generates this:
0000000000001000 <main>: 1000: f3 0f 1e fa endbr64 1004: 41 54 push %r12 1006: 41 bc cc 00 00 00 mov $0xcc,%r12d 100c: 55 push %rbp 100d: bd bb 00 00 00 mov $0xbb,%ebp 1012: 53 push %rbx 1013: bb aa 00 00 00 mov $0xaa,%ebx 1018: 90 nop 1019: b8 01 00 00 00 mov $0x1,%eax 101e: bf 01 00 00 00 mov $0x1,%edi 1023: ba 05 00 00 00 mov $0x5,%edx 1028: 48 8d 35 d1 0f 00 00 lea 0xfd1(%rip),%rsi 102f: 0f 05 syscall 1031: 90 nop 1032: 31 c0 xor %eax,%eax 1034: 5b pop %rbx 1035: 5d pop %rbp 1036: 41 5c pop %r12 1038: c3 ret
GCC thinks that syscall will clobber r8, r9, r10. So it spills 0xaa, 0xbb and 0xcc to callee saved registers (r12, rbp and rbx). This is clearly extra memory access and extra stack size for preserving them.
But syscall does not actually clobber them, so this is a missed optimization.
Now without r8, r9, r10 in the clobber list, GCC generates better code:
0000000000001000 <main>: 1000: f3 0f 1e fa endbr64 1004: 41 b8 aa 00 00 00 mov $0xaa,%r8d 100a: 41 b9 bb 00 00 00 mov $0xbb,%r9d 1010: 41 ba cc 00 00 00 mov $0xcc,%r10d 1016: 90 nop 1017: b8 01 00 00 00 mov $0x1,%eax 101c: bf 01 00 00 00 mov $0x1,%edi 1021: ba 05 00 00 00 mov $0x5,%edx 1026: 48 8d 35 d3 0f 00 00 lea 0xfd3(%rip),%rsi 102d: 0f 05 syscall 102f: 90 nop 1030: 31 c0 xor %eax,%eax 1032: c3 ret
Cc: Andy Lutomirski luto@kernel.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Ingo Molnar mingo@redhat.com Cc: Borislav Petkov bp@alien8.de Cc: x86@kernel.org Cc: "H. Peter Anvin" hpa@zytor.com Cc: David Laight David.Laight@ACULAB.COM Acked-by: Andy Lutomirski luto@kernel.org Signed-off-by: Ammar Faizi ammar.faizi@students.amikom.ac.id Link: https://gitlab.com/x86-psABIs/x86-64-ABI/-/wikis/x86-64-psABI [1] Link: https://lore.kernel.org/lkml/20211011040344.437264-1-ammar.faizi@students.am... Signed-off-by: Willy Tarreau w@1wt.eu Signed-off-by: Paul E. McKenney paulmck@kernel.org Stable-dep-of: 184177c3d6e0 ("tools/nolibc: restore mips branch ordering in the _start block") Signed-off-by: Sasha Levin sashal@kernel.org --- tools/include/nolibc/nolibc.h | 33 +++++++++++++++++++-------------- 1 file changed, 19 insertions(+), 14 deletions(-)
diff --git a/tools/include/nolibc/nolibc.h b/tools/include/nolibc/nolibc.h index d64020c1922c..ece7a70d8b39 100644 --- a/tools/include/nolibc/nolibc.h +++ b/tools/include/nolibc/nolibc.h @@ -265,12 +265,17 @@ struct stat { * - arguments are in rdi, rsi, rdx, r10, r8, r9 respectively * - the system call is performed by calling the syscall instruction * - syscall return comes in rax - * - rcx and r8..r11 may be clobbered, others are preserved. + * - rcx and r11 are clobbered, others are preserved. * - the arguments are cast to long and assigned into the target registers * which are then simply passed as registers to the asm code, so that we * don't have to experience issues with register constraints. * - the syscall number is always specified last in order to allow to force * some registers before (gcc refuses a %-register at the last position). + * - see also x86-64 ABI section A.2 AMD64 Linux Kernel Conventions, A.2.1 + * Calling Conventions. + * + * Link x86-64 ABI: https://gitlab.com/x86-psABIs/x86-64-ABI/-/wikis/x86-64-psABI + * */
#define my_syscall0(num) \ @@ -280,9 +285,9 @@ struct stat { \ asm volatile ( \ "syscall\n" \ - : "=a" (_ret) \ + : "=a"(_ret) \ : "0"(_num) \ - : "rcx", "r8", "r9", "r10", "r11", "memory", "cc" \ + : "rcx", "r11", "memory", "cc" \ ); \ _ret; \ }) @@ -295,10 +300,10 @@ struct stat { \ asm volatile ( \ "syscall\n" \ - : "=a" (_ret) \ + : "=a"(_ret) \ : "r"(_arg1), \ "0"(_num) \ - : "rcx", "r8", "r9", "r10", "r11", "memory", "cc" \ + : "rcx", "r11", "memory", "cc" \ ); \ _ret; \ }) @@ -312,10 +317,10 @@ struct stat { \ asm volatile ( \ "syscall\n" \ - : "=a" (_ret) \ + : "=a"(_ret) \ : "r"(_arg1), "r"(_arg2), \ "0"(_num) \ - : "rcx", "r8", "r9", "r10", "r11", "memory", "cc" \ + : "rcx", "r11", "memory", "cc" \ ); \ _ret; \ }) @@ -330,10 +335,10 @@ struct stat { \ asm volatile ( \ "syscall\n" \ - : "=a" (_ret) \ + : "=a"(_ret) \ : "r"(_arg1), "r"(_arg2), "r"(_arg3), \ "0"(_num) \ - : "rcx", "r8", "r9", "r10", "r11", "memory", "cc" \ + : "rcx", "r11", "memory", "cc" \ ); \ _ret; \ }) @@ -349,10 +354,10 @@ struct stat { \ asm volatile ( \ "syscall\n" \ - : "=a" (_ret), "=r"(_arg4) \ + : "=a"(_ret) \ : "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4), \ "0"(_num) \ - : "rcx", "r8", "r9", "r11", "memory", "cc" \ + : "rcx", "r11", "memory", "cc" \ ); \ _ret; \ }) @@ -369,10 +374,10 @@ struct stat { \ asm volatile ( \ "syscall\n" \ - : "=a" (_ret), "=r"(_arg4), "=r"(_arg5) \ + : "=a"(_ret) \ : "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4), "r"(_arg5), \ "0"(_num) \ - : "rcx", "r9", "r11", "memory", "cc" \ + : "rcx", "r11", "memory", "cc" \ ); \ _ret; \ }) @@ -390,7 +395,7 @@ struct stat { \ asm volatile ( \ "syscall\n" \ - : "=a" (_ret), "=r"(_arg4), "=r"(_arg5) \ + : "=a"(_ret) \ : "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4), "r"(_arg5), \ "r"(_arg6), "0"(_num) \ : "rcx", "r11", "memory", "cc" \