powerpc/vdso: Avoid link stack corruption in __get_datapage()

powerpc has a link register (lr) used for calling functions. We "bl
<func>" to call a function, and "blr" to return back to the call site.

The lr is only a single register, so if we call another function from
inside this function (ie. nested calls), software must save away the
lr on the software stack before calling the new function. Before
returning (ie. before the "blr"), the lr is restored by software from
the software stack.

This makes branch prediction quite difficult for the processor as it
will only know the branch target just before the "blr".

To help with this, modern powerpc processors keep a (non-architected)
hardware stack of lr called a "link stack". When a "bl <func>" is
run, the lr is pushed onto this stack. When a "blr" is called, the
branch predictor pops the lr value from the top of the link stack, and
uses it to predict the branch target. Hence the processor pipeline
knows a lot earlier the branch target.

This works great but there are some cases where you call "bl" but
without a matching "blr". Once such case is when trying to determine
the program counter (which can't be read directly). Here you "bl+4;
mflr" to get the program counter. If you do this, the link stack will
get out of sync with reality, causing the branch predictor to
mis-predict subsequent function returns.

To avoid this, modern micro-architectures have a special case of bl.
Using the form "bcl 20,31,+4", ensures the processor doesn't push to
the link stack.

The 32 and 64 bit variants of __get_datapage() use a "bl; mflr" to
determine the loaded address of the VDSO. The current versions of
these attempt to use this special bl variant.

Unfortunately they use +8 rather than the required +4. Hence the
current code results in the link stack getting out of sync with
reality and hence the resulting performance degradation.

This patch moves it to bcl+4 by moving __kernel_datapage_offset out of
__get_datapage().

With this patch, running a gettimeofday() (which uses
__get_datapage()) microbenchmark we get a decent bump in performance
on POWER7/8.

For the benchmark in tools/testing/selftests/powerpc/benchmarks/gettimeofday.c
  POWER8:
    64bit gets ~4% improvement
    32bit gets ~9% improvement
  POWER7:
    64bit gets ~7% improvement

Signed-off-by: Michael Neuling <mikey@neuling.org>
Reported-by: Aaron Sawdey <sawdey@us.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --git a/arch/powerpc/kernel/vdso32/datapage.S b/arch/powerpc/kernel/vdso32/datapage.S
index dc21e891d2e71b12ebc1dfb709357689dc089fd2..59cf5f452879bef8729a7b58797694b6adac1f49 100644 (file)
--- a/arch/powerpc/kernel/vdso32/datapage.S
+++ b/arch/powerpc/kernel/vdso32/datapage.S
@@ -16,6 +16,10 @@
 #include <asm/vdso.h>
 
        .text
+       .global __kernel_datapage_offset;
+__kernel_datapage_offset:
+       .long   0
+
 V_FUNCTION_BEGIN(__get_datapage)
   .cfi_startproc
        /* We don't want that exposed or overridable as we want other objects
@@ -27,13 +31,11 @@ V_FUNCTION_BEGIN(__get_datapage)
        mflr    r0
   .cfi_register lr,r0
 
-       bcl     20,31,1f
-       .global __kernel_datapage_offset;
-__kernel_datapage_offset:
-       .long   0
-1:
+       bcl     20,31,data_page_branch
+data_page_branch:
        mflr    r3
        mtlr    r0
+       addi    r3, r3, __kernel_datapage_offset-data_page_branch
        lwz     r0,0(r3)
        add     r3,r0,r3
        blr

diff --git a/arch/powerpc/kernel/vdso64/datapage.S b/arch/powerpc/kernel/vdso64/datapage.S
index 79796de1173743d20d34806609680c4967d94839..2f01c4a0d8a037ca65cacd9af51022846661fcb0 100644 (file)
--- a/arch/powerpc/kernel/vdso64/datapage.S
+++ b/arch/powerpc/kernel/vdso64/datapage.S
@@ -16,6 +16,10 @@
 #include <asm/vdso.h>
 
        .text
+.global        __kernel_datapage_offset;
+__kernel_datapage_offset:
+       .long   0
+
 V_FUNCTION_BEGIN(__get_datapage)
   .cfi_startproc
        /* We don't want that exposed or overridable as we want other objects
@@ -27,13 +31,11 @@ V_FUNCTION_BEGIN(__get_datapage)
        mflr    r0
   .cfi_register lr,r0
 
-       bcl     20,31,1f
-       .global __kernel_datapage_offset;
-__kernel_datapage_offset:
-       .long   0
-1:
+       bcl     20,31,data_page_branch
+data_page_branch:
        mflr    r3
        mtlr    r0
+       addi    r3, r3, __kernel_datapage_offset-data_page_branch
        lwz     r0,0(r3)
        add     r3,r0,r3
        blr