Re: [PATCH] perf: arm_spe: Add barrier before enabling profiling buffer

[James Clark <james.clark@linaro.org>]

On 19/02/2026 12:57 pm, Will Deacon wrote:
> On Thu, Feb 19, 2026 at 12:08:27PM +0000, James Clark wrote:
>> I'm back to drag this up again. So I think all of the above discussion
>> relies on the ordering given by the indirect read needed for the "might
>> ignore a direct write..." part. But it's _might_ ignore a direct write, it's
>> possible for an implementation to not do that, so there are two possible
>> implementations:
>>
>> #1 Where there is an indirect read to give the write ignore outcome
>> #2 Where there is no write ignore outcome so it doesn't require an
>>     indirect read
>>
>> For #2 there's nothing to force the ordering. We're writing to two different
>> registers (PMBPTR_EL1 and PMBLIMITR_EL1) and we have to have the
>> PMBLIMITR_EL1 write come second for the buffer to be considered configured
>> correctly. For example if the old value of PMBPTR_EL1 is higher than the new
>> PMBLIMITR_EL1 and the write to PMBLIMITR_EL1 happens first then it's
>> misconfigured. That's why we think we need the isb() here.
> 
> I thought profiling was disabled in these cases, so why is it
> misconfigured?
> 
> If it is misconfigured, what can go wrong given that we're either stopped
> or pmscr is clear?
> 
> Will

It is stopped in the interrupt handler because PMBSR_EL1.S = 1. But in 
arm_spe_pmu_start(), PMBSR_EL1.S = 0 so it's not stopped. And with 
PMBPTR_EL1 still being set to the value from the last session it could 
be higher than PMBLIMITR_EL1.

PMSCR_EL1 doesn't affect SPEProfilingStopped(), it's only:

   boolean stopped = (PMBSR_EL1.S == '1');

The conditions for when the buffer needs to be configured correctly from 
R24557 are:

   ProfilingBufferEnabled() && !SPEProfilingStopped() &&
     PMBLIMITR_EL1.FM != DISCARD

I think even if PMSCR_EL1 is clear you can still get a buffer management 
error, even if no samples were going to be written into the buffer. It 
just says:

   While the Profiling Buffer is enabled, profiling is not stopped, and
   Discard mode is not enabled, all of the following must be true:

   * The current write pointer must be at least one sample record below
     the write limit pointer.