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

[Will Deacon <will@kernel.org>]

On Tue, Feb 03, 2026 at 10:46:37AM +0000, James Clark wrote:
> 
> 
> 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.

My reading of the the text above is that it is covering the direct write
-> indirect read case, whereas I think the case in the SPE driver that
we're considering for your patch is when we have an indirect read
followed by a direct write.

Will