[RFCv1 4/4] perf: arm_spe: Dynamically switch PID tracing to contextidr

[James Clark]

On 21/10/2021 14:45, Leo Yan wrote:
> Now Arm64 provides API for enabling and disable PID tracing, Arm SPE
> driver invokes these functions to dynamically enable it during
> profiling when the program runs in root PID name space, and disable PID
> tracing when the perf event is stopped.
> 
> Device drivers should not depend on CONFIG_PID_IN_CONTEXTIDR for PID
> tracing, so this patch uses the consistent condition for setting bit
> EL1_CX for PMSCR.

Hi Leo,

I've been testing this change, but I'm seeing something strange. Not sure
if it's a problem on my side or not yet. With this command:

 sudo ./perf record -vvv -e arm_spe//u -- taskset --cpu-list 1 bash -c ls

I'm only seeing 0 values for context:

 sudo ./perf report -D | grep CONTEXT

.  00038dce:  65 00 00 00 00                                  CONTEXT 0x0 el2
.  00038e0e:  65 00 00 00 00                                  CONTEXT 0x0 el2

I added a printk to the function, and I see it print non zero values, although
there are some zero ones mixed in there too:

 diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
index 0c1669db19a1..8f0fb43a5fac 100644
--- a/arch/arm64/include/asm/mmu_context.h
+++ b/arch/arm64/include/asm/mmu_context.h
@@ -33,7 +33,8 @@ static inline void contextidr_thread_switch(struct task_struct *next)
        if (!static_branch_unlikely(&contextidr_in_use))
                return;
 
-       write_sysreg(task_pid_nr(next), contextidr_el1);
+       printk("Set %d\n", task_pid_nr(next));
+       write_sysreg(task_pid_nr(next), contextidr_el2);
        isb();
 }
 

Results in this:

[   53.257905] Set 77
[   53.257909] Set 0
[   53.258180] Set 77
[   53.258183] Set 0
[   53.258385] Set 309
[   53.258385] Set 172
[   53.258425] Set 77
[   53.258443] Set 990
[   53.258449] Set 77
[   53.258455] Set 990
[   53.258467] Set 310
[   53.258719] Set 7
[   53.258728] Set 77
[   53.258731] Set 0
[   53.258733] Set 0
[   53.258738] Set 7


Without your patchset I don't get 0 values in the SPE trace anymore:

.  0000050e:  65 b1 01 00 00                                  CONTEXT 0x1b1 el2
.  0000054e:  65 b1 01 00 00                                  CONTEXT 0x1b1 el2
.  0000058e:  65 ac 01 00 00                                  CONTEXT 0x1ac el2
.  000005ce:  65 ac 01 00 00                                  CONTEXT 0x1ac el2