Re: [PATCH v4 0/4] arm64: cross-CPU NMI via SDEI

Hi Kirill
On Mon, Jun 22, 2026 at 02:56:16PM +0100, Kiryl Shutsemau wrote:
> On Fri, Jun 19, 2026 at 03:26:21PM +0100, Marc Zyngier wrote:
> > > Does your firmware set ICC_CTLR_EL1.PMHE? I'd be curious to see the
> > > numbers if the DSB was omitted on the enable path.
> >
> > I certainly don't observe this sort of overhead on the HW I have
> > access to, and would like to understand where this is coming from with
> > actual profiling data.
> 
> Full disclosure: the ~66% figures come from internal testing about a year ago.
> I no longer have the details of the machine it ran on and can't confirm whether
> ICC_CTLR_EL1.PMHE was set there -- it may well have been. I shouldn't have
> carried those numbers forward without being able to stand behind them, so
> please disregard them.
> 
> Here are fresh numbers from NVIDIA Grace (Neoverse V2). Importantly, this
> box reports:
> 
>   GICv3: Pseudo-NMIs enabled using relaxed ICC_PMR_EL1 synchronisation
> 
> i.e. PMHE == 0, so the synchronising DSB on the unmask path is already
> patched to a NOP (ARM64_HAS_GIC_PRIO_RELAXED_SYNC). What's left is the
> floor cost of PMR-based masking itself plus the PMR save/restore on
> exception entry/exit -- not the DSB. So this is the case Catalin asked
> about (DSB omitted), and there is still a measurable cost.
> 
> A trivial single-threaded gettid() loop (1e6 calls, median of 5,
> performance governor, ASLR off):
> 
>   pseudo_nmi=0 (DAIF):       178.4 ns/call
>   pseudo_nmi=1 (PMR):        252.5 ns/call
>   delta:                     +74.1 ns/call  (~230-250 cycles)
>                              +41.5% wall time / 0.706 throughput
I tested the u-bench.c on a Neoverse N2 based arm64 server. The result
is as following:
   pseudo_nmi=0 (DAIF):       96.3  ns/call
   pseudo_nmi=1 (PMR):        169.8 ns/call
   delta:                     +73.5 ns/call

> 
>   --- u-bench.c ---
>   #include <unistd.h>
>   #include <sys/syscall.h>
>   #include <time.h>
>   #include <stdio.h>
>   int main(void) {
>           struct timespec a, b;
>           clock_gettime(CLOCK_MONOTONIC, &a);
>           for (long i = 0; i < 1000000; i++)
>                   syscall(SYS_gettid);
>           clock_gettime(CLOCK_MONOTONIC, &b);
>           printf("%f ns\n", (b.tv_sec-a.tv_sec)*1e9 + (b.tv_nsec-a.tv_nsec));
>           return 0;
>   }
> 
> will-it-scale agrees independently. sched_yield (ops/s, median of 5):
> 
>                       1 task       72 tasks
>   pseudo_nmi=0      3,195,656    230,824,534
>   pseudo_nmi=1      2,253,753    163,914,837
>   ratio                0.705          0.710
> 
> The ratio is flat across the whole 1-to-72 sweep, so -- relevant to the
> scalability question -- it's a constant per-syscall tax, not a contention
> effect. The impact tracks syscall/exception density: page_fault1, a more
> realistic workload, stays within ~5%.
> 
> > The direction of travel is to deprecate SDEI. I wouldn't add more stuff
> > on top of this interface.
> 
> I understand FEAT_NMI is the long-term answer, and I'm not arguing against
> deprecating SDEI. My concern is the gap in between. By our estimate it's
> 10+ years before the last non-FEAT_NMI machine retires from the fleet --
> for scale, we're still running Skylake today. So there's roughly a
> decade where a large installed base has neither FEAT_NMI nor affordable
> pseudo-NMI, and no way to reach a DAIF-masked CPU for an all-CPU
> backtrace or to capture a wedged CPU in a crash dump. That's the
> functional gap this series tries to cover.
> 
> Given the deprecation direction, I deliberately kept the SDEI footprint as
> small as I could. The series adds no new firmware interface and no vendor
> SMC -- it uses only the standard software-signalled event (event 0) via
> SDEI_EVENT_SIGNAL, which is already present on these systems for
> firmware-first RAS (APEI/GHES). And SDEI is only ever invoked in a "bad
> state": to deliver a backtrace signal to a CPU that a normal IPI can't
> reach, or to stop a CPU that ignored the stop IPIs. Nothing on any hot or
> steady-state path touches it.
> 
> If even that minimal use is unacceptable on a deprecated interface, I'd
> rather know now and redirect the effort -- but I'd appreciate a pointer to
> what should cover this gap for existing silicon in the meantime.
I couldn't agree more: We need a solution for existing system. And
like to see this patchset merged. Thanks.

Regards
Yin, Fengwei
> 
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov
>