[RFC 2/7] x86/percpu: Use compiler segment prefix qualifier

From: Nadav Amit
Date: Thu Jul 18 2019 - 21:04:07 EST

Using a segment prefix qualifier is cleaner than using a segment prefix
in the inline assembly, and provides the compiler with more information,
telling it that __seg_gs:[addr] is different than [addr] when it
analyzes data dependencies. It also enables various optimizations that
will be implemented in the next patches.

Use segment prefix qualifiers when they are supported. Unfortunately,
gcc does not provide a way to remove segment qualifiers, which is needed
to use typeof() to create local instances of the per-cpu variable. For
this reason, do not use the segment qualifier for per-cpu variables, and
do casting using the segment qualifier instead.

Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
arch/x86/include/asm/percpu.h | 153 ++++++++++++++++++++++------------
1 file changed, 102 insertions(+), 51 deletions(-)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 2278797c769d..1fe348884477 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -45,8 +45,33 @@
#include <linux/kernel.h>
#include <linux/stringify.h>

+#if defined(COMPILER_HAS_X86_SEG_SUPPORT) && IS_ENABLED(CONFIG_SMP)
+#define USE_X86_SEG_SUPPORT 1
+#else
+#define USE_X86_SEG_SUPPORT 0
+#endif
+
#ifdef CONFIG_SMP
+
+#if USE_X86_SEG_SUPPORT
+
+#ifdef CONFIG_X86_64
+#define __percpu_seg_override __seg_gs
+#else
+#define __percpu_seg_override __seg_fs
+#endif
+
+#define __percpu_prefix ""
+#define __force_percpu_prefix "%%"__stringify(__percpu_seg)":"
+
+#else /* USE_X86_SEG_SUPPORT */
+
+#define __percpu_seg_override
#define __percpu_prefix "%%"__stringify(__percpu_seg)":"
+#define __force_percpu_prefix __percpu_prefix
+
+#endif /* USE_X86_SEG_SUPPORT */
+
#define __my_cpu_offset this_cpu_read(this_cpu_off)

/*
@@ -58,14 +83,21 @@
unsigned long tcp_ptr__; \
asm volatile("add " __percpu_arg(1) ", %0" \
: "=r" (tcp_ptr__) \
- : "m" (this_cpu_off), "0" (ptr)); \
+ : "m" (__my_cpu_var(this_cpu_off)),\
+ "0" (ptr)); \
(typeof(*(ptr)) __kernel __force *)tcp_ptr__; \
})
-#else
+#else /* CONFIG_SMP */
+#define __percpu_seg_override
#define __percpu_prefix ""
-#endif
+#define __force_percpu_prefix ""
+#endif /* CONFIG_SMP */

+#define __my_cpu_type(var) typeof(var) __percpu_seg_override
+#define __my_cpu_ptr(ptr) (__my_cpu_type(*ptr) *)(uintptr_t)(ptr)
+#define __my_cpu_var(var) (*__my_cpu_ptr(&var))
#define __percpu_arg(x) __percpu_prefix "%" #x
+#define __force_percpu_arg(x) __force_percpu_prefix "%" #x

/*
* Initialized pointers to per-cpu variables needed for the boot
@@ -98,22 +130,22 @@ do { \
switch (sizeof(var)) { \
case 1: \
asm qual (op "b %1,"__percpu_arg(0) \
- : "+m" (var) \
+ : "+m" (__my_cpu_var(var)) \
: "qi" ((pto_T__)(val))); \
break; \
case 2: \
asm qual (op "w %1,"__percpu_arg(0) \
- : "+m" (var) \
+ : "+m" (__my_cpu_var(var)) \
: "ri" ((pto_T__)(val))); \
break; \
case 4: \
asm qual (op "l %1,"__percpu_arg(0) \
- : "+m" (var) \
+ : "+m" (__my_cpu_var(var)) \
: "ri" ((pto_T__)(val))); \
break; \
case 8: \
asm qual (op "q %1,"__percpu_arg(0) \
- : "+m" (var) \
+ : "+m" (__my_cpu_var(var)) \
: "re" ((pto_T__)(val))); \
break; \
default: __bad_percpu_size(); \
@@ -138,71 +170,79 @@ do { \
switch (sizeof(var)) { \
case 1: \
if (pao_ID__ == 1) \
- asm qual ("incb "__percpu_arg(0) : "+m" (var)); \
+ asm qual ("incb "__percpu_arg(0) : \
+ "+m" (__my_cpu_var(var))); \
else if (pao_ID__ == -1) \
- asm qual ("decb "__percpu_arg(0) : "+m" (var)); \
+ asm qual ("decb "__percpu_arg(0) : \
+ "+m" (__my_cpu_var(var))); \
else \
asm qual ("addb %1, "__percpu_arg(0) \
- : "+m" (var) \
+ : "+m" (__my_cpu_var(var)) \
: "qi" ((pao_T__)(val))); \
break; \
case 2: \
if (pao_ID__ == 1) \
- asm qual ("incw "__percpu_arg(0) : "+m" (var)); \
+ asm qual ("incw "__percpu_arg(0) : \
+ "+m" (__my_cpu_var(var))); \
else if (pao_ID__ == -1) \
- asm qual ("decw "__percpu_arg(0) : "+m" (var)); \
+ asm qual ("decw "__percpu_arg(0) : \
+ "+m" (__my_cpu_var(var))); \
else \
asm qual ("addw %1, "__percpu_arg(0) \
- : "+m" (var) \
+ : "+m" (__my_cpu_var(var)) \
: "ri" ((pao_T__)(val))); \
break; \
case 4: \
if (pao_ID__ == 1) \
- asm qual ("incl "__percpu_arg(0) : "+m" (var)); \
+ asm qual ("incl "__percpu_arg(0) : \
+ "+m" (__my_cpu_var(var))); \
else if (pao_ID__ == -1) \
- asm qual ("decl "__percpu_arg(0) : "+m" (var)); \
+ asm qual ("decl "__percpu_arg(0) : \
+ "+m" (__my_cpu_var(var))); \
else \
asm qual ("addl %1, "__percpu_arg(0) \
- : "+m" (var) \
+ : "+m" (__my_cpu_var(var)) \
: "ri" ((pao_T__)(val))); \
break; \
case 8: \
if (pao_ID__ == 1) \
- asm qual ("incq "__percpu_arg(0) : "+m" (var)); \
+ asm qual ("incq "__percpu_arg(0) : \
+ "+m" (__my_cpu_var(var))); \
else if (pao_ID__ == -1) \
- asm qual ("decq "__percpu_arg(0) : "+m" (var)); \
+ asm qual ("decq "__percpu_arg(0) : \
+ "+m" (__my_cpu_var(var))); \
else \
asm qual ("addq %1, "__percpu_arg(0) \
- : "+m" (var) \
+ : "+m" (__my_cpu_var(var)) \
: "re" ((pao_T__)(val))); \
break; \
default: __bad_percpu_size(); \
} \
} while (0)

-#define percpu_from_op(qual, op, var) \
-({ \
+#define percpu_from_op(qual, op, var) \
+({ \
typeof(var) pfo_ret__; \
switch (sizeof(var)) { \
case 1: \
asm qual (op "b "__percpu_arg(1)",%0" \
: "=q" (pfo_ret__) \
- : "m" (var)); \
+ : "m" (__my_cpu_var(var))); \
break; \
case 2: \
asm qual (op "w "__percpu_arg(1)",%0" \
: "=r" (pfo_ret__) \
- : "m" (var)); \
+ : "m" (__my_cpu_var(var))); \
break; \
case 4: \
asm qual (op "l "__percpu_arg(1)",%0" \
: "=r" (pfo_ret__) \
- : "m" (var)); \
+ : "m" (__my_cpu_var(var))); \
break; \
case 8: \
asm qual (op "q "__percpu_arg(1)",%0" \
: "=r" (pfo_ret__) \
- : "m" (var)); \
+ : "m" (__my_cpu_var(var))); \
break; \
default: __bad_percpu_size(); \
} \
@@ -214,22 +254,22 @@ do { \
typeof(var) pfo_ret__; \
switch (sizeof(var)) { \
case 1: \
- asm(op "b "__percpu_arg(P1)",%0" \
+ asm(op "b "__force_percpu_arg(P1)",%0" \
: "=q" (pfo_ret__) \
: "p" (&(var))); \
break; \
case 2: \
- asm(op "w "__percpu_arg(P1)",%0" \
+ asm(op "w "__force_percpu_arg(P1)",%0" \
: "=r" (pfo_ret__) \
: "p" (&(var))); \
break; \
case 4: \
- asm(op "l "__percpu_arg(P1)",%0" \
+ asm(op "l "__force_percpu_arg(P1)",%0" \
: "=r" (pfo_ret__) \
: "p" (&(var))); \
break; \
case 8: \
- asm(op "q "__percpu_arg(P1)",%0" \
+ asm(op "q "__force_percpu_arg(P1)",%0" \
: "=r" (pfo_ret__) \
: "p" (&(var))); \
break; \
@@ -243,19 +283,19 @@ do { \
switch (sizeof(var)) { \
case 1: \
asm qual (op "b "__percpu_arg(0) \
- : "+m" (var)); \
+ : "+m" (__my_cpu_var(var))); \
break; \
case 2: \
asm qual (op "w "__percpu_arg(0) \
- : "+m" (var)); \
+ : "+m" (__my_cpu_var(var))); \
break; \
case 4: \
asm qual (op "l "__percpu_arg(0) \
- : "+m" (var)); \
+ : "+m" (__my_cpu_var(var))); \
break; \
case 8: \
asm qual (op "q "__percpu_arg(0) \
- : "+m" (var)); \
+ : "+m" (__my_cpu_var(var))); \
break; \
default: __bad_percpu_size(); \
} \
@@ -270,22 +310,26 @@ do { \
switch (sizeof(var)) { \
case 1: \
asm qual ("xaddb %0, "__percpu_arg(1) \
- : "+q" (paro_ret__), "+m" (var) \
+ : "+q" (paro_ret__), \
+ "+m" (__my_cpu_var(var)) \
: : "memory"); \
break; \
case 2: \
asm qual ("xaddw %0, "__percpu_arg(1) \
- : "+r" (paro_ret__), "+m" (var) \
+ : "+r" (paro_ret__), \
+ "+m" (__my_cpu_var(var)) \
: : "memory"); \
break; \
case 4: \
asm qual ("xaddl %0, "__percpu_arg(1) \
- : "+r" (paro_ret__), "+m" (var) \
+ : "+r" (paro_ret__), \
+ "+m" (__my_cpu_var(var)) \
: : "memory"); \
break; \
case 8: \
asm qual ("xaddq %0, "__percpu_arg(1) \
- : "+re" (paro_ret__), "+m" (var) \
+ : "+re" (paro_ret__), \
+ "+m" (__my_cpu_var(var)) \
: : "memory"); \
break; \
default: __bad_percpu_size(); \
@@ -308,7 +352,8 @@ do { \
asm qual ("\n\tmov "__percpu_arg(1)",%%al" \
"\n1:\tcmpxchgb %2, "__percpu_arg(1) \
"\n\tjnz 1b" \
- : "=&a" (pxo_ret__), "+m" (var) \
+ : "=&a" (pxo_ret__), \
+ "+m" (__my_cpu_var(var)) \
: "q" (pxo_new__) \
: "memory"); \
break; \
@@ -316,7 +361,8 @@ do { \
asm qual ("\n\tmov "__percpu_arg(1)",%%ax" \
"\n1:\tcmpxchgw %2, "__percpu_arg(1) \
"\n\tjnz 1b" \
- : "=&a" (pxo_ret__), "+m" (var) \
+ : "=&a" (pxo_ret__), \
+ "+m" (__my_cpu_var(var)) \
: "r" (pxo_new__) \
: "memory"); \
break; \
@@ -324,7 +370,8 @@ do { \
asm qual ("\n\tmov "__percpu_arg(1)",%%eax" \
"\n1:\tcmpxchgl %2, "__percpu_arg(1) \
"\n\tjnz 1b" \
- : "=&a" (pxo_ret__), "+m" (var) \
+ : "=&a" (pxo_ret__), \
+ "+m" (__my_cpu_var(var)) \
: "r" (pxo_new__) \
: "memory"); \
break; \
@@ -332,7 +379,8 @@ do { \
asm qual ("\n\tmov "__percpu_arg(1)",%%rax" \
"\n1:\tcmpxchgq %2, "__percpu_arg(1) \
"\n\tjnz 1b" \
- : "=&a" (pxo_ret__), "+m" (var) \
+ : "=&a" (pxo_ret__), \
+ "+m" (__my_cpu_var(var)) \
: "r" (pxo_new__) \
: "memory"); \
break; \
@@ -353,25 +401,25 @@ do { \
switch (sizeof(var)) { \
case 1: \
asm qual ("cmpxchgb %2, "__percpu_arg(1) \
- : "=a" (pco_ret__), "+m" (var) \
+ : "=a" (pco_ret__), "+m" (__my_cpu_var(var))\
: "q" (pco_new__), "0" (pco_old__) \
: "memory"); \
break; \
case 2: \
asm qual ("cmpxchgw %2, "__percpu_arg(1) \
- : "=a" (pco_ret__), "+m" (var) \
+ : "=a" (pco_ret__), "+m" (__my_cpu_var(var))\
: "r" (pco_new__), "0" (pco_old__) \
: "memory"); \
break; \
case 4: \
asm qual ("cmpxchgl %2, "__percpu_arg(1) \
- : "=a" (pco_ret__), "+m" (var) \
+ : "=a" (pco_ret__), "+m" (__my_cpu_var(var))\
: "r" (pco_new__), "0" (pco_old__) \
: "memory"); \
break; \
case 8: \
asm qual ("cmpxchgq %2, "__percpu_arg(1) \
- : "=a" (pco_ret__), "+m" (var) \
+ : "=a" (pco_ret__), "+m" (__my_cpu_var(var))\
: "r" (pco_new__), "0" (pco_old__) \
: "memory"); \
break; \
@@ -464,7 +512,8 @@ do { \
typeof(pcp2) __o2 = (o2), __n2 = (n2); \
asm volatile("cmpxchg8b "__percpu_arg(1) \
CC_SET(z) \
- : CC_OUT(z) (__ret), "+m" (pcp1), "+m" (pcp2), "+a" (__o1), "+d" (__o2) \
+ : CC_OUT(z) (__ret), "+m" (__my_cpu_var(pcp1)), \
+ "+m" (__my_cpu_var(pcp2)), "+a" (__o1), "+d" (__o2) \
: "b" (__n1), "c" (__n2)); \
__ret; \
})
@@ -507,11 +556,13 @@ do { \
bool __ret; \
typeof(pcp1) __o1 = (o1), __n1 = (n1); \
typeof(pcp2) __o2 = (o2), __n2 = (n2); \
- alternative_io("leaq %P1,%%rsi\n\tcall this_cpu_cmpxchg16b_emu\n\t", \
- "cmpxchg16b " __percpu_arg(1) "\n\tsetz %0\n\t", \
+ alternative_io("leaq %P[ptr],%%rsi\n\tcall this_cpu_cmpxchg16b_emu\n\t", \
+ "cmpxchg16b " __force_percpu_arg([ptr]) "\n\tsetz %0\n\t",\
X86_FEATURE_CX16, \
- ASM_OUTPUT2("=a" (__ret), "+m" (pcp1), \
- "+m" (pcp2), "+d" (__o2)), \
+ ASM_OUTPUT2("=a" (__ret), \
+ "+m" (__my_cpu_var(pcp1)), \
+ "+m" (__my_cpu_var(pcp2)), \
+ "+d" (__o2)), [ptr]"m" (pcp1), \
"b" (__n1), "c" (__n2), "a" (__o1) : "rsi"); \
__ret; \
})
@@ -542,7 +593,7 @@ static inline bool x86_this_cpu_variable_test_bit(int nr,
asm volatile("btl "__percpu_arg(2)",%1"
CC_SET(c)
: CC_OUT(c) (oldbit)
- : "m" (*(unsigned long __percpu *)addr), "Ir" (nr));
+ : "m" (*__my_cpu_ptr((unsigned long __percpu *)(addr))), "Ir" (nr));

return oldbit;
}
--
2.17.1