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

[Leo Yan <leo.yan@arm.com>]

On Mon, Feb 02, 2026 at 06:57:11PM +0000, Will Deacon wrote:

[...]


> > > I'm not sure I follow your logic as to why both ISBs are required, but
> > > I'd have thought that if perf_aux_output_begin() fails when called from
> > > arm_spe_perf_aux_output_begin() in the irqhandler, we need the ISB
> > > because we're going to clear pmblimitr_el1 to 0 and that surely has
> > > to be ordered before clearing pmbsr?
> > 
> > I think the ISB after arm_spe_perf_aux_output_begin() in the irq
> > handler is required for both the failure and success cases.
> > 
> > For a normal maintenance interrupt, an ISB is inserted between writing
> > PMBLIMITR_EL1 and PMBSR_EL1 to ensure that a valid limit write is
> > visible before tracing restarts.  This ensures that the following
> > conditions are safely met:
> > 
> >   "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.
> > 
> >    PMBPTR_EL1.PTR[63:56] must equal PMBLIMITR_EL1.LIMIT[63:56],
> >    regardless of the value of the applicable TBI bit."
> 
> Hmm, so let's say we've executed the first ISB. At that point, the
> Profiling Buffer is disabled (PMBLIMITR_EL1.E = 0) and profiling is
> stopped (PMBSR_EL1.S = 1).

This is not true.  PMBLIMITR_EL1.E is always 1 during interrupt
handling.

> If we *don't* have the second ISB then either
> PMBLIMITR_EL1 is written first or PMBSR_EL1 is written first. But the
> text you quoted will only come into effect once they've both happened,
> right? In which case, why does the order matter for the success case?

Yes, both PMBLIMITR_EL1.E == 1 and PMBSR_EL1.S == 0 must be true to
enable tracing.

However, the tricky part is that PMBLIMITR_EL1.E remains 1 throughout
the sequence.  Writing PMBLIMITR_EL1 effectively only sets the limit,
while clearing PMBSR_EL1 is the distinct step that enables tracing.

Thanks,
Leo