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

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

On 02/02/2026 7:03 pm, Will Deacon wrote:
> On Fri, Jan 23, 2026 at 04:03:53PM +0000, James Clark wrote:
>> The Arm ARM known issues document [1] states that the architecture will
>> be relaxed so that the profiling buffer must be correctly configured
>> when ProfilingBufferEnabled() && !SPEProfilingStopped() &&
>> PMBLIMITR_EL1.FM != DISCARD:
>>
>>    R24557
>>
>>    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.
>>
>> The same relaxation also says that writes may be completely ignored:
>>
>>    When the Profiling Buffer is enabled, profiling is not stopped, and
>>    Discard mode is not enabled, the PE might ignore a direct write to any
>>    of the following Profiling Buffer registers, other than a direct write
>>    to PMBLIMITR_EL1 that clears PMBLIMITR_EL1.E from 1 to 0:
>>
>>    * The current write pointer, PMBPTR_EL1.
>>    * The Limit pointer, PMBLIMITR_EL1.
>>    * PMBSR_EL1.
> 
> Thinking about this some more, does that mean that the direct write to
> PMBPTR_EL1 performs an indirect read of PMBLIMITR_EL1 so that it can
> determine the write-ignore semantics? If so, doesn't that mean that
> we'll get order against a subsequent direct write of PMBLIMITR_EL1
> without an ISB thanks to table "D24-1 Synchronization requirements"
> which says that an indirect read followed by a direct write doesn't
> require synchronisation?
> 
> There's also a sentence above the table stating:
> 
> "Direct writes to System registers are not allowed to affect any
>   instructions appearing in program order before the direct write."
> 
> so after all that, I'm not really sure why the ISB is required.
> 
> Will

We were under the impression that this was required for the SPU as it is 
treated as a separate entity than the PE.

In "D17.9 Synchronization and Statistical Profiling" there is:

   INDWCG

   A Context Synchronization event guarantees that a direct write to a
   System register made by the PE in program order before the Context
   synchronization event are observable by indirect reads and indirect
   writes of the same System register made by a profiling operation
   relating to a sampled operation in program order after the Context
   synchronization event.

That specifically mentions an indirect read following a direct write, 
which seems to contradict D24-1. Although I thought this is a special 
case for SPE.