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

From: James Clark

Date: Tue Feb 03 2026 - 04:30:13 EST

On 02/02/2026 7:14 pm, Leo Yan wrote:

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

I think Leo is correct that the old isb() is still needed. I removed it under the assumption that PMBLIMITR_EL1.E was unset in the interrupt handler. Possibly because the previous version re-arranged the handler to do that.

If PMBLIMITR_EL1.E is set, we have to make sure clearing PMBSR_EL1 comes last as it's the thing that defines the point where both pointers must be correct by.