Re: [PATCH] arm64: smp: distinguish secondary CPUs that hang after reaching head.S

[Will Deacon]

On Fri, Jul 24, 2026 at 10:16:03AM +0530, Naman Jain wrote:
> On 7/23/2026 11:26 AM, Anshuman Khandual wrote:
> > On 23/07/26 8:19 AM, Jinjie Ruan wrote:
> > > 在 2026/7/22 19:30, Naman Jain 写道:
> > > > When a secondary CPU fails to come online, __cpu_up() falls back to
> > > > __early_cpu_boot_status, but boot status 0x0 is ambiguous: it cannot
> > > > distinguish a CPU that never executed head.S (firmware/hypervisor never
> > > > dispatched it, so it never ran a single instruction) from one that
> > > > entered head.S, started executing, and then got stuck somewhere in kernel
> > > > bring-up. Add a change to let us tell those two cases apart, which
> > > > narrows down where to look when a CPU goes missing during boot.
> > > I previously encountered this issue when debugging the parallel startup
> > > of ARM64 secondary cores. It is difficult for the kernel to determine
> > > whether the secondary core is hung in the firmware or whether it has not
> > > executed a single instruction. So I think this motive is reasonable.
> > 
> > Why should kernel determine the difference here ? Would not the firmware
> > know if it has started any secondary CPU for the kernel which must have
> > come inside head.S ? If the cpu gets hung inside firmware while starting
> > up then the debug responsibilities belong there instead.
> > 
> > Still wondering what's the rationale for this change.
> 
> Hello Anshuman,
> This sounds fair to me. Let me elaborate the problem, beyond the scope of
> this patch. In production, we occasionally see these crashes where one of
> the CPU fails to bring up online, with 0x0 status code. Hypervisor may be
> missing the telemetry, but the problem is that we don't know if the
> secondary CPU ever started executing the instructions or is stuck somewhere
> between the start of head.S and marking itself online at the end of
> secondary_start_kernel().
> There are couple of places, where we get those other status codes, but not
> everywhere. If the issue is not easily reproducible, experiments on local
> setups do not yield anything. That's where I am attempting to add some more
> information in kernel to debug these issues.

I think this is a game of diminishing returns. There's a lot of stuff
that the firmware/hypervisor can get wrong here and trying to detect or
handle that in Linux is going to be a real mess. For example, if it
enters the kernel at the wrong address, or in the wrong mode, or with
the MMU enabled etc. It sounds like you don't have much idea about
what happens in the failure case, so it might not even execute the code
that you're adding correctly.

It's also going to conflict heavily with the ongoing parallel bringup
work, which significantly reworks this code.

I think you need to add the debug to your hypervisor, rather than the
kernel.

Will