[External] Re: [PATCH v3 5/8] riscv: smp: use NMI for CPU stop

[yunhui cui]

Hi Radim,

On Thu, Dec 4, 2025 at 9:16 PM Radim Krčmář <rkrcmar at ventanamicro.com> wrote:
>
> 2025-12-04T13:28:45+08:00, yunhui cui <cuiyunhui at bytedance.com>:
> > Hi Radim,
> >
> > On Thu, Dec 4, 2025 at 12:07 PM Radim Krčmář <rkrcmar at ventanamicro.com> wrote:
> >>
> >> 2025-11-27T20:53:02+08:00, Yunhui Cui <cuiyunhui at bytedance.com>:
> >> > Use NMI instead of IPI for CPU stop if RISC-V SSE NMI is supported.
> >> >
> >> > Signed-off-by: Yunhui Cui <cuiyunhui at bytedance.com>
> >> > ---
> >> > diff --git a/drivers/firmware/riscv/riscv_sse_nmi.c b/drivers/firmware/riscv/riscv_sse_nmi.c
> >> > @@ -58,6 +58,7 @@ static int local_nmi_handler(u32 evt, void *arg, struct pt_regs *regs)
> >> >       type = atomic_read(this_cpu_ptr(&local_nmi));
> >> >
> >> >       NMI_HANDLE(LOCAL_NMI_CRASH, cpu_crash_stop, cpu, regs);
> >> > +     NMI_HANDLE(LOCAL_NMI_STOP, cpu_stop);
> >>
> >> Please document the intended preemption design for all SSE events,
> >> because it will be a nightmare if we forget some assumptions in the
> >> coming years.  (That includes the relative priorities of RAS/PMU/...)
> >
> > Actually, LOCAL_NMI_CRASH, LOCAL_NMI_STOP, LOCAL_NMI_BACKTRACE,
> > LOCAL_NMI_KGDB, ... are all implemented via the single SSE event
> > SBI_SSE_EVENT_LOCAL_SOFTWARE_INJECTED. Per the SSE design, no
> > preemption will occur among CRASH, STOP, BACKTRACE, and KGDB events.
>
> That is how it is.  I don't understand why it must be like that.
>
> For example: PMU_OVERFLOW has lower event_id than SOFTWARE_INJECTED, so
> it will currently interrupt NMI_CRASH as they both have priority 0,
> although NMI_CRASH probably shouldn't be masked by anything, and should
> preempt everything.
> NMI_BACKTRACE, on the other hand, probably shouldn't have that high
> priority as there seem more important events (e.g. RAS and NMI_CRASH).
>
> The issues can be avoided by event priorities, masking, or deemed as
> non-issue, but I think it would be beneficial to provide some reasoning
> behind the design, as the choices don't seem obvious to me.

Indeed, it is necessary to consider the priority among different
events. Should different priorities also be assigned to NMI_CRASH,
NMI_BACKTRACE, NMI_STOP, and NMI_KGDB? Do these operations need to be
visible to the BIOS? Could you kindly provide some good suggestions?

>
> Thanks.

Thanks,
Yunhui