Re: [PATCH] x86/ucode: Remove MICROCODE_UPDATE_TIMEOUT and associated panic()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.08.2026 13:13, Andrew Cooper wrote:
> On 18/08/2026 3:31 pm, Jan Beulich wrote:
>> On 18.08.2026 15:07, Andrew Cooper wrote:
>>> Panicing in the case of a timeout turns out to be about the worst possible
>>> action Xen can take.  It leaves all other APs waiting on the condition
>>> variable, some in NMI context.  As a result, they fail to be shot down and
>>> dump state for kexec crash analysis.
>> At the same time there likely isn't much to be learned from a kexec dump, as
>> the source of the issue is in the CPU, not in Xen.
> 
> The single most valuable print message I've ever added to Xen is the one
> which reports which CPUs didn't respond to NMIs.  Up until now, it has
> always highlighted hardware issues.
> 
> Despite my wish to remove this panic specifically, there is still
> information to be gained from kexec in a similar scenario.

That is you think of a system without console, where those log messages would
only be possible to fish out of the dump. Fair enough.

>> Further, this code runs with the watchdog disabled. If there's truly no
>> progress anymore, how would one know from the outside whether the system is
>> dead altogether vs the control CPU still kicking around?
> 
> The scenario you describe can only occur if the BSP accepts the
> microcode successfully, and one of the APs locks up properly.
> 
> If the BSP locks up, we never get as far as deciding to panic(), and the
> system hangs already.
> 
> If we have a bad microcode, it is far more likely for the BSP to hang
> than for the BSP to work one of the APs hang.

Hmm, probably. (I'm always having in mind the one old system I have where only
the primary cores on each socket get ucode updated by firmware, with secondary
cores needing us to deal with them.) Still somewhat hesitantly:
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

>>> Microcode Loading on Granite Rapids takes about 4.5s of wallclock time, far 
>>> in
>>> excess of the of the arbitrary 1s Xen allows.  This time is spent in the 
>>> WRMSR
>>> to load the blob, and there's nothing the system can do but to sit and wait.
>>> Despite the delay, the system as a whole does survive.
>> For this I wonder whether the log message issues after 1 sec is adequate. If
>> we know things can take this long, wouldn't we better issue the log message
>> no earlier than, say, 5s * nr_sockets?
> 
> I have some work there not submitted yet, which would periodically print
> a message.  That at least gives you slight signs of life.
> 
> But nothing involving nr_sockets.  It's far more often wrong than it is
> right, and that's not how our algorithm scales.
> 
> GNR has gone from milliseconds to 4.5s.  Putting the limit at 5s is just
> going to need another change in a year or two.

That's one possibility, yes. The other is that they realize that they have to
bring processing time back down.

Jan