Hi Raphael,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on linus/master]
[cannot apply to v5.3-rc4 next-20190816]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Raphael-Gault/perf-arm64-Add-test-to-check-userspace-access-to-hardware-counters/20190818-182238
config: arm-omap2plus_defconfig (attached as .config)
compiler: arm-linux-gnueabi-gcc (GCC) 7.4.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        GCC_VERSION=7.4.0 make.cross ARCH=arm 

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <redacted>

All errors (new ones prefixed by >>):

   drivers/perf/arm_pmu.c: In function 'refresh_pmuserenr':

quoted

drivers/perf/arm_pmu.c:784:2: error: implicit declaration of function 'perf_switch_user_access'; did you mean 'perf_fetch_caller_regs'? [-Werror=implicit-function-declaration]

     perf_switch_user_access(mm);
     ^~~~~~~~~~~~~~~~~~~~~~~
     perf_fetch_caller_regs
   drivers/perf/arm_pmu.c: In function 'armpmu_event_mapped':

quoted

drivers/perf/arm_pmu.c:804:36: error: 'mm_context_t {aka struct <anonymous>}' has no member named 'pmu_direct_access'

     if (atomic_inc_return(&mm->context.pmu_direct_access) == 1)
                                       ^
   drivers/perf/arm_pmu.c: In function 'armpmu_event_unmapped':
   drivers/perf/arm_pmu.c:813:38: error: 'mm_context_t {aka struct <anonymous>}' has no member named 'pmu_direct_access'
     if (atomic_dec_and_test(&mm->context.pmu_direct_access))
                                         ^
   cc1: some warnings being treated as errors

vim +784 drivers/perf/arm_pmu.c

   781	
   782	static void refresh_pmuserenr(void *mm)
   783	{
 > 784		perf_switch_user_access(mm);
   785	}
   786	
   787	static void armpmu_event_mapped(struct perf_event *event, struct mm_struct *mm)
   788	{
   789		if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
   790			return;
   791	
   792		/*
   793		 * This function relies on not being called concurrently in two
   794		 * tasks in the same mm.  Otherwise one task could observe
   795		 * pmu_direct_access > 1 and return all the way back to
   796		 * userspace with user access disabled while another task is still
   797		 * doing on_each_cpu_mask() to enable user access.
   798		 *
   799		 * For now, this can't happen because all callers hold mmap_sem
   800		 * for write.  If this changes, we'll need a different solution.
   801		 */
   802		lockdep_assert_held_write(&mm->mmap_sem);
   803	
 > 804		if (atomic_inc_return(&mm->context.pmu_direct_access) == 1)
   805			on_each_cpu(refresh_pmuserenr, mm, 1);
   806	}
   807	

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