[PATCH v2 0/2] lib: sbi: pmu: Rework SSE callbacks

[Samuel Holland]

Hi Clément,

On 2025-05-19 3:39 AM, Clément Léger wrote:
> While testing the PMU SSE support in Linux, a few problem were spotted.
> The first one was due to interrupts happing during some PMU critical
> sections. Indeed, the PMU core actually disable interrupts when entering
> these sections. However, while this works when interrupts are targeting
> the S-mode, this is not the case with SSE. Fixing this required to
> implement pmu_disable/enable() in Linux [1]. These two callbacks now
> call sse_event_disable/enable() which now just needs to set/clear
> MIE/MIP. Second problem was that reported PMU events count were quite
> low. This proved to be due to some interrupts being losed when
> reenabling SSE event. We still need to clear the pending PMU interrupts
> upon disabling otherwise we might get some spurious interrupts when
> reenabling it.

Could you clarify what could cause spurious interrupts here? Disabling the SSE
event should just prevent it from being delivered to S-mode, right? It doesn't
affect the event's pending status. My reading of the privileged spec is that
mip.LCOFIP is edge-triggered by the individual hpmcounter OF bits. So if any
counter is counting, and mip.LCOFIP is cleared without handing the overflow,
S-mode would (erroneously) lose an overflow event. This could happen if for
example an overflow occurs and pmu_sse_disable() was called both from within the
SSE handler.

If my understanding is correct, to match the edge interrupt semantics (clear
LCOFIP before handling, as noted in the privileged spec), I think the only place
that should clear mip.LCOFIP is sbi_pmu_ovf_irq(), not sbi_pmu_ctr_stop() or
pmu_sse_disable().

Regards,
Samuel

> Link: https://lore.kernel.org/linux-riscv/20250516152355.560448-1-cleger@rivosinc.com/ [1]
> 
> Clément Léger (2):
>   lib: sbi: pmu: Add SSE register/unregister() callbacks
>   lib: sbi: pmu: Remove MIP clearing from pmu_sse_enable()
> 
>  lib/sbi/sbi_pmu.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
>