[PATCH -tip v3 3/3] x86/percpu: Introduce %rip-relative addressing to PER_CPU_VAR

[Uros Bizjak <ubizjak@xxxxxxxxx>]

Introduce x86_64 %rip-relative addressing to PER_CPU_VAR macro.
Instructions using %rip-relative address operand are one byte shorter
than their absolute address counterparts and are also compatible with
position independent executable (-fpie) build. The patch reduces
code size of a test kernel build by 150 bytes.

PER_CPU_VAR macro is intended to be applied to a symbol and should
not be used with register operands. Introduce new __percpu macro and
use it in cmpxchg{8,16}b_emu.S instead.

Also add a missing function comment to this_cpu_cmpxchg8b_emu.

No functional changes intended.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
--
v2: Introduce PER_CPU_ARG macro to conditionally enable
    segment registers in cmpxchg{8,16}b_emu.S for CONFIG_SMP.
v3: Introduce __percpu macro instead of PER_CPU_ARG (hpa).
---
 arch/x86/include/asm/percpu.h | 12 ++++++++----
 arch/x86/lib/cmpxchg16b_emu.S | 12 ++++++------
 arch/x86/lib/cmpxchg8b_emu.S  | 30 +++++++++++++++++++++---------
 3 files changed, 35 insertions(+), 19 deletions(-)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 54746903b8c3..02f1780f02f5 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -4,17 +4,21 @@
 
 #ifdef CONFIG_X86_64
 #define __percpu_seg           gs
+#define __percpu_rel           (%rip)
 #else
 #define __percpu_seg           fs
+#define __percpu_rel
 #endif
 
 #ifdef __ASSEMBLY__
 
 #ifdef CONFIG_SMP
-#define PER_CPU_VAR(var)       %__percpu_seg:var
-#else /* ! SMP */
-#define PER_CPU_VAR(var)       var
-#endif /* SMP */
+#define __percpu               %__percpu_seg:
+#else
+#define __percpu
+#endif
+
+#define PER_CPU_VAR(var)       __percpu(var)__percpu_rel
 
 #ifdef CONFIG_X86_64_SMP
 #define INIT_PER_CPU_VAR(var)  init_per_cpu__##var
diff --git a/arch/x86/lib/cmpxchg16b_emu.S b/arch/x86/lib/cmpxchg16b_emu.S
index 6962df315793..4fb44894ad87 100644
--- a/arch/x86/lib/cmpxchg16b_emu.S
+++ b/arch/x86/lib/cmpxchg16b_emu.S
@@ -23,14 +23,14 @@ SYM_FUNC_START(this_cpu_cmpxchg16b_emu)
        cli
 
        /* if (*ptr == old) */
-       cmpq    PER_CPU_VAR(0(%rsi)), %rax
+       cmpq    __percpu (%rsi), %rax
        jne     .Lnot_same
-       cmpq    PER_CPU_VAR(8(%rsi)), %rdx
+       cmpq    __percpu 8(%rsi), %rdx
        jne     .Lnot_same
 
        /* *ptr = new */
-       movq    %rbx, PER_CPU_VAR(0(%rsi))
-       movq    %rcx, PER_CPU_VAR(8(%rsi))
+       movq    %rbx, __percpu (%rsi)
+       movq    %rcx, __percpu 8(%rsi)
 
        /* set ZF in EFLAGS to indicate success */
        orl     $X86_EFLAGS_ZF, (%rsp)
@@ -42,8 +42,8 @@ SYM_FUNC_START(this_cpu_cmpxchg16b_emu)
        /* *ptr != old */
 
        /* old = *ptr */
-       movq    PER_CPU_VAR(0(%rsi)), %rax
-       movq    PER_CPU_VAR(8(%rsi)), %rdx
+       movq    __percpu (%rsi), %rax
+       movq    __percpu 8(%rsi), %rdx
 
        /* clear ZF in EFLAGS to indicate failure */
        andl    $(~X86_EFLAGS_ZF), (%rsp)
diff --git a/arch/x86/lib/cmpxchg8b_emu.S b/arch/x86/lib/cmpxchg8b_emu.S
index 873e4ef23e49..1c96be769adc 100644
--- a/arch/x86/lib/cmpxchg8b_emu.S
+++ b/arch/x86/lib/cmpxchg8b_emu.S
@@ -24,12 +24,12 @@ SYM_FUNC_START(cmpxchg8b_emu)
        pushfl
        cli
 
-       cmpl    0(%esi), %eax
+       cmpl    (%esi), %eax
        jne     .Lnot_same
        cmpl    4(%esi), %edx
        jne     .Lnot_same
 
-       movl    %ebx, 0(%esi)
+       movl    %ebx, (%esi)
        movl    %ecx, 4(%esi)
 
        orl     $X86_EFLAGS_ZF, (%esp)
@@ -38,7 +38,7 @@ SYM_FUNC_START(cmpxchg8b_emu)
        RET
 
 .Lnot_same:
-       movl    0(%esi), %eax
+       movl    (%esi), %eax
        movl    4(%esi), %edx
 
        andl    $(~X86_EFLAGS_ZF), (%esp)
@@ -53,18 +53,30 @@ EXPORT_SYMBOL(cmpxchg8b_emu)
 
 #ifndef CONFIG_UML
 
+/*
+ * Emulate 'cmpxchg8b %fs:(%rsi)'
+ *
+ * Inputs:
+ * %esi : memory location to compare
+ * %eax : low 32 bits of old value
+ * %edx : high 32 bits of old value
+ * %ebx : low 32 bits of new value
+ * %ecx : high 32 bits of new value
+ *
+ * Notably this is not LOCK prefixed and is not safe against NMIs
+ */
 SYM_FUNC_START(this_cpu_cmpxchg8b_emu)
 
        pushfl
        cli
 
-       cmpl    PER_CPU_VAR(0(%esi)), %eax
+       cmpl    __percpu (%esi), %eax
        jne     .Lnot_same2
-       cmpl    PER_CPU_VAR(4(%esi)), %edx
+       cmpl    __percpu 4(%esi), %edx
        jne     .Lnot_same2
 
-       movl    %ebx, PER_CPU_VAR(0(%esi))
-       movl    %ecx, PER_CPU_VAR(4(%esi))
+       movl    %ebx, __percpu (%esi)
+       movl    %ecx, __percpu 4(%esi)
 
        orl     $X86_EFLAGS_ZF, (%esp)
 
@@ -72,8 +84,8 @@ SYM_FUNC_START(this_cpu_cmpxchg8b_emu)
        RET
 
 .Lnot_same2:
-       movl    PER_CPU_VAR(0(%esi)), %eax
-       movl    PER_CPU_VAR(4(%esi)), %edx
+       movl    __percpu (%esi), %eax
+       movl    __percpu 4(%esi), %edx
 
        andl    $(~X86_EFLAGS_ZF), (%esp)
 
-- 
2.41.0