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

[Denis Kirjanov]

On 9/24/15, Michael Ellerman <michael at ellerman.id.au> wrote:
>
>
> On 23 September 2015 16:05:02 GMT+10:00, Michael Neuling <mikey at neuling.org>
> wrote:
>>The 32 and 64 bit variants of __get_datapage() use a "bcl; mflr" to
>>determine the loaded address of the VDSO. The current version of these
>>attempt to use the special bcl variant which avoids pushing to the
>>link stack.
>>
>>Unfortunately it uses bcl+8 rather than the required bcl+4. Hence the
>>current code results in link stack corruption and the resulting
>>performance degradation (due to branch mis-prediction).
>>
>>This patch moves us to bcl+4 by moving __kernel_datapage_offset
>>out of __get_datapage().
>>
>>With this patch, running the below benchmark we get a bump in
>>performance on POWER8 for gettimeofday() (which uses
>>__get_datapage()).
>>
>>64bit gets ~4% improvement:
>>  Without patch:
>>    # ./tb
>>    time = 0.180321
>>  With patch:
>>    # ./tb
>>    time = 0.187408
>>
>>32bit gets ~9% improvement:
>>  Without patch:
>>    # ./tb
>>    time = 0.276551
>>  With patch:
>>    # ./tb
>>    time = 0.252767
>>
>>Testcase tb.c (stolen from Anton)
>>  /* gcc -O2 tb.c -o tb */
>>  #include <sys/time.h>
>>  #include <stdio.h>
>>
>>  int main()
>>  {
>>	  int i;
>>
>>	  struct timeval tv_start, tv_end;
>>
>>	  gettimeofday(&tv_start, NULL);
>>
>>	  for(i = 0; i < 10000000; i++) {
>>		  gettimeofday(&tv_end, NULL);
>>	  }
>>
>>	  printf("time = %.6f\n", tv_end.tv_sec - tv_start.tv_sec +
>>(tv_end.tv_usec - tv_start.tv_usec) * 1e-6);
>>
>>	  return 0;
>>  }
>
> You know where test cases are supposed to go.
>
> I know it's not a pass/fail test, but it's still useful. If it's in the tree
> it will get run as part of automated test runs and we will have a record of
> the result over time.

I can send a patch for it.

>
> cheers
> --
> Sent from my Android phone with K-9 Mail. Please excuse my brevity.
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