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

[Michael Ellerman]

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.

cheers
-- 
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