[PATCH 2/2] x86-64: use RIP-relative addressing for most per-CPU accesses

From: Jan Beulich
Date: Tue Nov 04 2014 - 03:50:58 EST

Observing that per-CPU data (in the SMP case) is reachable by
exploiting 64-bit address wraparound (building on the default kernel
load address being at 16Mb), the one byte shorter RIP-relative
addressing form can be used for most per-CPU accesses. The one
exception are the "stable" reads, where the use of the "P" operand
modifier prevents the compiler from using RIP-relative addressing, but
is unavoidable due to the use of the "p" constraint (side note: with
gcc 4.9.x the intended effect of this isn't being achieved anymore,
see gcc bug 63637).

With the dependency on the minimum kernel load address, arbitrarily
low values for CONFIG_PHYSICAL_START are now no longer possible. A
link time assertion is being added, directing to the need to increase
that value when it triggers.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
arch/x86/include/asm/percpu.h | 59 +++++++++++++++++++++++++++++++-----------
arch/x86/kernel/vmlinux.lds.S | 2 +
2 files changed, 46 insertions(+), 15 deletions(-)

--- 3.18-rc3-x86_64-PCrel-per-CPU.orig/arch/x86/include/asm/percpu.h
+++ 3.18-rc3-x86_64-PCrel-per-CPU/arch/x86/include/asm/percpu.h
@@ -64,7 +64,7 @@
#define __percpu_prefix ""
#endif

-#define __percpu_arg(x) __percpu_prefix "%P" #x
+#define __percpu_arg(x) __percpu_prefix "%" #x

/*
* Initialized pointers to per-cpu variables needed for the boot
@@ -179,29 +179,58 @@ do { \
} \
} while (0)

-#define percpu_from_op(op, var, constraint) \
+#define percpu_from_op(op, var) \
({ \
typeof(var) pfo_ret__; \
switch (sizeof(var)) { \
case 1: \
asm(op "b "__percpu_arg(1)",%0" \
: "=q" (pfo_ret__) \
- : constraint); \
+ : "m" (var)); \
break; \
case 2: \
asm(op "w "__percpu_arg(1)",%0" \
: "=r" (pfo_ret__) \
- : constraint); \
+ : "m" (var)); \
break; \
case 4: \
asm(op "l "__percpu_arg(1)",%0" \
: "=r" (pfo_ret__) \
- : constraint); \
+ : "m" (var)); \
break; \
case 8: \
asm(op "q "__percpu_arg(1)",%0" \
: "=r" (pfo_ret__) \
- : constraint); \
+ : "m" (var)); \
+ break; \
+ default: __bad_percpu_size(); \
+ } \
+ pfo_ret__; \
+})
+
+#define percpu_stable_op(op, var) \
+({ \
+ typeof(var) pfo_ret__; \
+ switch (sizeof(var)) { \
+ case 1: \
+ asm(op "b "__percpu_arg(P1)",%0" \
+ : "=q" (pfo_ret__) \
+ : "p" (&(var))); \
+ break; \
+ case 2: \
+ asm(op "w "__percpu_arg(P1)",%0" \
+ : "=r" (pfo_ret__) \
+ : "p" (&(var))); \
+ break; \
+ case 4: \
+ asm(op "l "__percpu_arg(P1)",%0" \
+ : "=r" (pfo_ret__) \
+ : "p" (&(var))); \
+ break; \
+ case 8: \
+ asm(op "q "__percpu_arg(P1)",%0" \
+ : "=r" (pfo_ret__) \
+ : "p" (&(var))); \
break; \
default: __bad_percpu_size(); \
} \
@@ -359,11 +388,11 @@ do { \
* per-thread variables implemented as per-cpu variables and thus
* stable for the duration of the respective task.
*/
-#define this_cpu_read_stable(var) percpu_from_op("mov", var, "p" (&(var)))
+#define this_cpu_read_stable(var) percpu_stable_op("mov", var)

-#define raw_cpu_read_1(pcp) percpu_from_op("mov", (pcp), "m"(pcp))
-#define raw_cpu_read_2(pcp) percpu_from_op("mov", (pcp), "m"(pcp))
-#define raw_cpu_read_4(pcp) percpu_from_op("mov", (pcp), "m"(pcp))
+#define raw_cpu_read_1(pcp) percpu_from_op("mov", pcp)
+#define raw_cpu_read_2(pcp) percpu_from_op("mov", pcp)
+#define raw_cpu_read_4(pcp) percpu_from_op("mov", pcp)

#define raw_cpu_write_1(pcp, val) percpu_to_op("mov", (pcp), val)
#define raw_cpu_write_2(pcp, val) percpu_to_op("mov", (pcp), val)
@@ -381,9 +410,9 @@ do { \
#define raw_cpu_xchg_2(pcp, val) percpu_xchg_op(pcp, val)
#define raw_cpu_xchg_4(pcp, val) percpu_xchg_op(pcp, val)

-#define this_cpu_read_1(pcp) percpu_from_op("mov", (pcp), "m"(pcp))
-#define this_cpu_read_2(pcp) percpu_from_op("mov", (pcp), "m"(pcp))
-#define this_cpu_read_4(pcp) percpu_from_op("mov", (pcp), "m"(pcp))
+#define this_cpu_read_1(pcp) percpu_from_op("mov", pcp)
+#define this_cpu_read_2(pcp) percpu_from_op("mov", pcp)
+#define this_cpu_read_4(pcp) percpu_from_op("mov", pcp)
#define this_cpu_write_1(pcp, val) percpu_to_op("mov", (pcp), val)
#define this_cpu_write_2(pcp, val) percpu_to_op("mov", (pcp), val)
#define this_cpu_write_4(pcp, val) percpu_to_op("mov", (pcp), val)
@@ -435,7 +464,7 @@ do { \
* 32 bit must fall back to generic operations.
*/
#ifdef CONFIG_X86_64
-#define raw_cpu_read_8(pcp) percpu_from_op("mov", (pcp), "m"(pcp))
+#define raw_cpu_read_8(pcp) percpu_from_op("mov", pcp)
#define raw_cpu_write_8(pcp, val) percpu_to_op("mov", (pcp), val)
#define raw_cpu_add_8(pcp, val) percpu_add_op((pcp), val)
#define raw_cpu_and_8(pcp, val) percpu_to_op("and", (pcp), val)
@@ -444,7 +473,7 @@ do { \
#define raw_cpu_xchg_8(pcp, nval) percpu_xchg_op(pcp, nval)
#define raw_cpu_cmpxchg_8(pcp, oval, nval) percpu_cmpxchg_op(pcp, oval, nval)

-#define this_cpu_read_8(pcp) percpu_from_op("mov", (pcp), "m"(pcp))
+#define this_cpu_read_8(pcp) percpu_from_op("mov", pcp)
#define this_cpu_write_8(pcp, val) percpu_to_op("mov", (pcp), val)
#define this_cpu_add_8(pcp, val) percpu_add_op((pcp), val)
#define this_cpu_and_8(pcp, val) percpu_to_op("and", (pcp), val)
--- 3.18-rc3-x86_64-PCrel-per-CPU.orig/arch/x86/kernel/vmlinux.lds.S
+++ 3.18-rc3-x86_64-PCrel-per-CPU/arch/x86/kernel/vmlinux.lds.S
@@ -186,6 +186,8 @@ SECTIONS
* start another segment - init.
*/
PERCPU_VADDR(INTERNODE_CACHE_BYTES, 0, :percpu)
+ ASSERT(SIZEOF(.data..percpu) < CONFIG_PHYSICAL_START,
+ "per-CPU data too large - increase CONFIG_PHYSICAL_START")
#endif

INIT_TEXT_SECTION(PAGE_SIZE)


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