linux-kernel - Re: [RFCv1 4/4] perf: arm_spe: Dynamically switch PID tracing to contextidr

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Message-ID: <854fb1a2-e5f1-f237-685f-8ddb0557c98b@arm.com>
Date:   Fri, 22 Oct 2021 16:36:05 +0100
From:   James Clark <james.clark@....com>
To:     Leo Yan <leo.yan@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Nicholas Piggin <npiggin@...il.com>,
        James Morse <james.morse@....com>,
        Marc Zyngier <maz@...nel.org>, Joey Gouly <joey.gouly@....com>,
        Peter Collingbourne <pcc@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Stephane Eranian <eranian@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFCv1 4/4] perf: arm_spe: Dynamically switch PID tracing to
 contextidr



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