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

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Message-Id: <DEPGLFUVHA0O.XSZVD2T1ENLD@ventanamicro.com>
Date: Thu, 04 Dec 2025 22:16:24 +0900
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "yunhui cui" <cuiyunhui@...edance.com>
Cc: <conor@...nel.org>, <paul.walmsley@...ive.com>, <palmer@...belt.com>,
 <aou@...s.berkeley.edu>, <alex@...ti.fr>, <luxu.kernel@...edance.com>,
 <linux-kernel@...r.kernel.org>, <linux-riscv@...ts.infradead.org>,
 <jassisinghbrar@...il.com>, <conor.dooley@...rochip.com>,
 <valentina.fernandezalanis@...rochip.com>, <catalin.marinas@....com>,
 <will@...nel.org>, <maz@...nel.org>, <timothy.hayes@....com>,
 <lpieralisi@...nel.org>, <arnd@...db.de>, <kees@...nel.org>,
 <tglx@...utronix.de>, <viresh.kumar@...aro.org>, <boqun.feng@...il.com>,
 <linux-arm-kernel@...ts.infradead.org>, <cleger@...osinc.com>,
 <atishp@...osinc.com>, <ajones@...tanamicro.com>, "linux-riscv"
 <linux-riscv-bounces@...ts.infradead.org>
Subject: Re: [External] Re: [PATCH v3 5/8] riscv: smp: use NMI for CPU stop

2025-12-04T13:28:45+08:00, yunhui cui <cuiyunhui@...edance.com>:
> Hi Radim,
>
> On Thu, Dec 4, 2025 at 12:07 PM Radim Krčmář <rkrcmar@...tanamicro.com> wrote:
>>
>> 2025-11-27T20:53:02+08:00, Yunhui Cui <cuiyunhui@...edance.com>:
>> > Use NMI instead of IPI for CPU stop if RISC-V SSE NMI is supported.
>> >
>> > Signed-off-by: Yunhui Cui <cuiyunhui@...edance.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.

Thanks.