[PATCH v4 4/4] arm64: escalate smp_send_stop() to an SDEI NMI as a last resort

[Doug Anderson]

Hi,

On Wed, Jun 17, 2026 at 12:20 PM Kiryl Shutsemau <kirill at shutemov.name> wrote:
>
> From: "Kiryl Shutsemau (Meta)" <kas at kernel.org>
>
> A CPU wedged with interrupts masked ignores the stop IPI, and without
> pseudo-NMI there is no NMI IPI to escalate to: a reboot proceeds with
> the CPU still running, and a kdump misses its registers.
>
> Add a third rung to smp_send_stop(): once the IPI (and pseudo-NMI IPI,
> if enabled) rungs have run, signal SDEI event 0 at whatever stayed
> online. Firmware delivers it regardless of the target's DAIF, so it
> reaches a CPU a plain IPI cannot; the target acks by going offline,
> which the caller already polls for.
>
> Fold the stop bookkeeping into one arm64_nmi_cpu_stop(regs,
> die_on_crash), shared by the stop IPI handlers, panic_smp_self_stop()
> and the SDEI handler, replacing the near-duplicate local_cpu_stop() and
> ipi_cpu_crash_stop(). @die_on_crash is the only difference: the IPI
> handlers pass true and PSCI CPU_OFF the CPU on a crash stop so a capture
> kernel can reclaim it; the SDEI handler and self-stop pass false and
> park. The SDEI park is required, not conservative -- its handler runs
> inside an SDEI event that is never completed (completing it resumes the
> wedged context), and a CPU_OFF from that unfinished-event context wedges
> EL3 on some firmware (left as a follow-up). The dump is unaffected; only
> re-onlining the CPU in an SMP capture kernel is lost.
>
> Suggested-by: Douglas Anderson <dianders at chromium.org>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas at kernel.org>
> ---
>  arch/arm64/include/asm/nmi.h    |  24 +++++++
>  arch/arm64/kernel/smp.c         | 113 +++++++++++++++++++++-----------
>  drivers/firmware/Kconfig        |   2 +
>  drivers/firmware/arm_sdei_nmi.c |  86 ++++++++++++++++++++++++
>  4 files changed, 187 insertions(+), 38 deletions(-)

Reviewed-by: Douglas Anderson <dianders at chromium.org>