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* [PATCH 0/3] nolibc updates and cleanups
@ 2021-10-24 17:43 Willy Tarreau
  2021-10-24 17:43 ` [PATCH 1/3] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list Willy Tarreau
                   ` (2 more replies)
  0 siblings, 3 replies; 4+ messages in thread
From: Willy Tarreau @ 2021-10-24 17:43 UTC (permalink / raw)
  To: Paul E . McKenney; +Cc: Ammar Faizi, linux-kernel

Hi Paul,

this small series includes the following updates for nolibc:

  - small cleanups and code reductions for x86_64 based on the removal
    of a few unneeded registers from the clobber list, and one shorter
    instruction to set the NR_exit value in %eax, both by Ammar Faizi.

  - Implementation of the gettid() syscall, by Mark Brown.

Please note, due to two adjacent instructions being touched by this
series and the previous one ("nolibc fixes marked for stable"), this
series will only apply after the stable one (I ordered the patches to
ease the backport to stable).

Thank you!
Willy

Ammar Faizi (2):
  tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list
  tools/nolibc: x86-64: Use `mov $60,%eax` instead of `mov $60,%rax`

Mark Brown (1):
  tools/nolibc: Implement gettid()

 tools/include/nolibc/nolibc.h | 53 +++++++++++++++++++++++++----------
 1 file changed, 38 insertions(+), 15 deletions(-)

-- 
2.17.5


^ permalink raw reply	[flat|nested] 4+ messages in thread
* [PATCH 1/3] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list
  2021-10-24 17:43 [PATCH 0/3] nolibc updates and cleanups Willy Tarreau
@ 2021-10-24 17:43 ` Willy Tarreau
  2021-10-24 17:43 ` [PATCH 2/3] tools/nolibc: x86-64: Use `mov $60,%eax` instead of `mov $60,%rax` Willy Tarreau
  2021-10-24 17:43 ` [PATCH 3/3] tools/nolibc: Implement gettid() Willy Tarreau
  2 siblings, 0 replies; 4+ messages in thread
From: Willy Tarreau @ 2021-10-24 17:43 UTC (permalink / raw)
  To: Paul E . McKenney
  Cc: Ammar Faizi, linux-kernel, Andy Lutomirski, Thomas Gleixner,
	Ingo Molnar, Borislav Petkov, x86, H. Peter Anvin, David Laight

From: Ammar Faizi <ammar.faizi@students.amikom.ac.id>

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.amikom.ac.id/
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
 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 3e2c6f2ed587..f9afe89ec6f2 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"                                \
-- 
2.17.5


^ permalink raw reply related	[flat|nested] 4+ messages in thread
* [PATCH 2/3] tools/nolibc: x86-64: Use `mov $60,%eax` instead of `mov $60,%rax`
  2021-10-24 17:43 [PATCH 0/3] nolibc updates and cleanups Willy Tarreau
  2021-10-24 17:43 ` [PATCH 1/3] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list Willy Tarreau
@ 2021-10-24 17:43 ` Willy Tarreau
  2021-10-24 17:43 ` [PATCH 3/3] tools/nolibc: Implement gettid() Willy Tarreau
  2 siblings, 0 replies; 4+ messages in thread
From: Willy Tarreau @ 2021-10-24 17:43 UTC (permalink / raw)
  To: Paul E . McKenney; +Cc: Ammar Faizi, linux-kernel

From: Ammar Faizi <ammar.faizi@students.amikom.ac.id>

Note that mov to 32-bit register will zero extend to 64-bit register.
Thus `mov $60,%eax` has the same effect with `mov $60,%rax`. Use the
shorter opcode to achieve the same thing.
```
  b8 3c 00 00 00       	mov    $60,%eax (5 bytes) [1]
  48 c7 c0 3c 00 00 00 	mov    $60,%rax (7 bytes) [2]
```
Currently, we use [2]. Change it to [1] for shorter code.

Signed-off-by: Ammar Faizi <ammar.faizi@students.amikom.ac.id>
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
 tools/include/nolibc/nolibc.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/include/nolibc/nolibc.h b/tools/include/nolibc/nolibc.h
index f9afe89ec6f2..4988866af0b5 100644
--- a/tools/include/nolibc/nolibc.h
+++ b/tools/include/nolibc/nolibc.h
@@ -420,7 +420,7 @@ asm(".section .text\n"
     "and $-16, %rsp\n"          // x86 ABI : esp must be 16-byte aligned before call
     "call main\n"               // main() returns the status code, we'll exit with it.
     "mov %eax, %edi\n"          // retrieve exit code (32 bit)
-    "mov $60, %rax\n"           // NR_exit == 60
+    "mov $60, %eax\n"           // NR_exit == 60
     "syscall\n"                 // really exit
     "hlt\n"                     // ensure it does not return
     "");
-- 
2.17.5


^ permalink raw reply related	[flat|nested] 4+ messages in thread
* [PATCH 3/3] tools/nolibc: Implement gettid()
  2021-10-24 17:43 [PATCH 0/3] nolibc updates and cleanups Willy Tarreau
  2021-10-24 17:43 ` [PATCH 1/3] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list Willy Tarreau
  2021-10-24 17:43 ` [PATCH 2/3] tools/nolibc: x86-64: Use `mov $60,%eax` instead of `mov $60,%rax` Willy Tarreau
@ 2021-10-24 17:43 ` Willy Tarreau
  2 siblings, 0 replies; 4+ messages in thread
From: Willy Tarreau @ 2021-10-24 17:43 UTC (permalink / raw)
  To: Paul E . McKenney; +Cc: Ammar Faizi, linux-kernel, Mark Brown

From: Mark Brown <broonie@kernel.org>

Allow test programs to determine their thread ID.

Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
 tools/include/nolibc/nolibc.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/tools/include/nolibc/nolibc.h b/tools/include/nolibc/nolibc.h
index 4988866af0b5..c1c285fe494a 100644
--- a/tools/include/nolibc/nolibc.h
+++ b/tools/include/nolibc/nolibc.h
@@ -1571,6 +1571,12 @@ pid_t sys_getpid(void)
 	return my_syscall0(__NR_getpid);
 }
 
+static __attribute__((unused))
+pid_t sys_gettid(void)
+{
+	return my_syscall0(__NR_gettid);
+}
+
 static __attribute__((unused))
 int sys_gettimeofday(struct timeval *tv, struct timezone *tz)
 {
@@ -2029,6 +2035,18 @@ pid_t getpid(void)
 	return ret;
 }
 
+static __attribute__((unused))
+pid_t gettid(void)
+{
+	pid_t ret = sys_gettid();
+
+	if (ret < 0) {
+		SET_ERRNO(-ret);
+		ret = -1;
+	}
+	return ret;
+}
+
 static __attribute__((unused))
 int gettimeofday(struct timeval *tv, struct timezone *tz)
 {
-- 
2.17.5


^ permalink raw reply related	[flat|nested] 4+ messages in thread
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Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-10-24 17:43 [PATCH 0/3] nolibc updates and cleanups Willy Tarreau
2021-10-24 17:43 ` [PATCH 1/3] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list Willy Tarreau
2021-10-24 17:43 ` [PATCH 2/3] tools/nolibc: x86-64: Use `mov $60,%eax` instead of `mov $60,%rax` Willy Tarreau
2021-10-24 17:43 ` [PATCH 3/3] tools/nolibc: Implement gettid() Willy Tarreau

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