powerpc: Feature nop out reservation clear when stcx checks address

The POWER architecture does not require stcx to check that it is operating
on the same address as the larx. This means it is possible for an
an exception handler to execute a larx, get a reservation, decide
not to do the stcx and then return back with an active reservation. If the
interrupted code was in the middle of a larx/stcx sequence the stcx could
incorrectly succeed.

All recent POWER CPUs check the address before letting the stcx succeed
so we can create a CPU feature and nop it out. As Ben suggested, we can
only do this in our syscall path because there is a remote possibility
some kernel code gets interrupted by an exception that ends up operating
on the same cacheline.

Thanks to Paul Mackerras and Derek Williams for the idea.

To test this I used a very simple null syscall (actually getppid) testcase
at http://ozlabs.org/~anton/junkcode/null_syscall.c

I tested against 2.6.35-git10 with the following changes against the
pseries_defconfig:

CONFIG_VIRT_CPU_ACCOUNTING=n
CONFIG_AUDIT=n
CONFIG_PPC_4K_PAGES=n
CONFIG_PPC_64K_PAGES=y
CONFIG_FORCE_MAX_ZONEORDER=9
CONFIG_PPC_SUBPAGE_PROT=n
CONFIG_FUNCTION_TRACER=n
CONFIG_FUNCTION_GRAPH_TRACER=n
CONFIG_IRQSOFF_TRACER=n
CONFIG_STACK_TRACER=n

to remove the overhead of virtual CPU accounting, syscall auditing and
the ftrace mcount tracers. 64kB pages were enabled to minimise TLB misses.

POWER6: +8.2%
POWER7: +7.0%

Another suggestion was to use a larx to something in the L1 instead of a stcx.
This was almost as fast as removing the larx on POWER6, but only 3.5% faster
on POWER7. We can use this to speed up the reservation clear in our
exception exit code.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

diff --git a/arch/powerpc/include/asm/cputable.h b/arch/powerpc/include/asm/cputable.h
index 3a40a99..f3a1fdd 100644 (file)
--- a/arch/powerpc/include/asm/cputable.h
+++ b/arch/powerpc/include/asm/cputable.h
@@ -198,6 +198,7 @@ extern const char *powerpc_base_platform;
 #define CPU_FTR_CP_USE_DCBTZ           LONG_ASM_CONST(0x0040000000000000)
 #define CPU_FTR_UNALIGNED_LD_STD       LONG_ASM_CONST(0x0080000000000000)
 #define CPU_FTR_ASYM_SMT               LONG_ASM_CONST(0x0100000000000000)
+#define CPU_FTR_STCX_CHECKS_ADDRESS    LONG_ASM_CONST(0x0200000000000000)
 
 #ifndef __ASSEMBLY__
 
@@ -392,28 +393,31 @@ extern const char *powerpc_base_platform;
            CPU_FTR_MMCRA | CPU_FTR_CTRL)
 #define CPU_FTRS_POWER4        (CPU_FTR_USE_TB | CPU_FTR_LWSYNC | \
            CPU_FTR_PPCAS_ARCH_V2 | CPU_FTR_CTRL | \
-           CPU_FTR_MMCRA | CPU_FTR_CP_USE_DCBTZ)
+           CPU_FTR_MMCRA | CPU_FTR_CP_USE_DCBTZ | \
+           CPU_FTR_STCX_CHECKS_ADDRESS)
 #define CPU_FTRS_PPC970        (CPU_FTR_USE_TB | CPU_FTR_LWSYNC | \
            CPU_FTR_PPCAS_ARCH_V2 | CPU_FTR_CTRL | \
            CPU_FTR_ALTIVEC_COMP | CPU_FTR_CAN_NAP | CPU_FTR_MMCRA | \
-           CPU_FTR_CP_USE_DCBTZ)
+           CPU_FTR_CP_USE_DCBTZ | CPU_FTR_STCX_CHECKS_ADDRESS)
 #define CPU_FTRS_POWER5        (CPU_FTR_USE_TB | CPU_FTR_LWSYNC | \
            CPU_FTR_PPCAS_ARCH_V2 | CPU_FTR_CTRL | \
            CPU_FTR_MMCRA | CPU_FTR_SMT | \
            CPU_FTR_COHERENT_ICACHE | CPU_FTR_LOCKLESS_TLBIE | \
-           CPU_FTR_PURR)
+           CPU_FTR_PURR | CPU_FTR_STCX_CHECKS_ADDRESS)
 #define CPU_FTRS_POWER6 (CPU_FTR_USE_TB | CPU_FTR_LWSYNC | \
            CPU_FTR_PPCAS_ARCH_V2 | CPU_FTR_CTRL | \
            CPU_FTR_MMCRA | CPU_FTR_SMT | \
            CPU_FTR_COHERENT_ICACHE | CPU_FTR_LOCKLESS_TLBIE | \
            CPU_FTR_PURR | CPU_FTR_SPURR | CPU_FTR_REAL_LE | \
-           CPU_FTR_DSCR | CPU_FTR_UNALIGNED_LD_STD)
+           CPU_FTR_DSCR | CPU_FTR_UNALIGNED_LD_STD | \
+           CPU_FTR_STCX_CHECKS_ADDRESS)
 #define CPU_FTRS_POWER7 (CPU_FTR_USE_TB | CPU_FTR_LWSYNC | \
            CPU_FTR_PPCAS_ARCH_V2 | CPU_FTR_CTRL | \
            CPU_FTR_MMCRA | CPU_FTR_SMT | \
            CPU_FTR_COHERENT_ICACHE | CPU_FTR_LOCKLESS_TLBIE | \
            CPU_FTR_PURR | CPU_FTR_SPURR | CPU_FTR_REAL_LE | \
-           CPU_FTR_DSCR | CPU_FTR_SAO  | CPU_FTR_ASYM_SMT)
+           CPU_FTR_DSCR | CPU_FTR_SAO  | CPU_FTR_ASYM_SMT | \
+           CPU_FTR_STCX_CHECKS_ADDRESS)
 #define CPU_FTRS_CELL  (CPU_FTR_USE_TB | CPU_FTR_LWSYNC | \
            CPU_FTR_PPCAS_ARCH_V2 | CPU_FTR_CTRL | \
            CPU_FTR_ALTIVEC_COMP | CPU_FTR_MMCRA | CPU_FTR_SMT | \

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 42e9d90..4d5fa12 100644 (file)
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -202,7 +202,9 @@ syscall_exit:
        bge-    syscall_error
 syscall_error_cont:
        ld      r7,_NIP(r1)
+BEGIN_FTR_SECTION
        stdcx.  r0,0,r1                 /* to clear the reservation */
+END_FTR_SECTION_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
        andi.   r6,r8,MSR_PR
        ld      r4,_LINK(r1)
        /*
@@ -419,6 +421,17 @@ END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
        sync
 #endif /* CONFIG_SMP */
 
+       /*
+        * If we optimise away the clear of the reservation in system
+        * calls because we know the CPU tracks the address of the
+        * reservation, then we need to clear it here to cover the
+        * case that the kernel context switch path has no larx
+        * instructions.
+        */
+BEGIN_FTR_SECTION
+       ldarx   r6,0,r1
+END_FTR_SECTION_IFSET(CPU_FTR_STCX_CHECKS_ADDRESS)
+
        addi    r6,r4,-THREAD   /* Convert THREAD to 'current' */
        std     r6,PACACURRENT(r13)     /* Set new 'current' */
 
@@ -576,7 +589,16 @@ ALT_FW_FTR_SECTION_END_IFCLR(FW_FEATURE_ISERIES)
        andi.   r0,r3,MSR_RI
        beq-    unrecov_restore
 
+       /*
+        * Clear the reservation. If we know the CPU tracks the address of
+        * the reservation then we can potentially save some cycles and use
+        * a larx. On POWER6 and POWER7 this is significantly faster.
+        */
+BEGIN_FTR_SECTION
        stdcx.  r0,0,r1         /* to clear the reservation */
+FTR_SECTION_ELSE
+       ldarx   r4,0,r1
+ALT_FTR_SECTION_END_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
 
        /*
         * Clear RI before restoring r13.  If we are returning to