Re: [PATCH] x86/nmi: Fix mis-classification of watchdog NMIs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.08.2026 19:43, Andrew Cooper wrote:
> It used to be the case that cpu_data[] inherited the BSP's cpuid_level until
> the AP had calculated it itself.  Following the rework, cpuid_level has a
> placeholder 1 until it is caluclated propely.
> 
> setup_apic_nmi_watchdog() happens to be called on the BSP after SMP bringup,
> meaning that the first call is on CPU1.  It is also positioned in the window
> where cpu_data[] is garbage.
> 
> As a result, setup_p6_watchdog()'s one-time calculation of the performance
> counter width falls back into Pentium compatibility mode assuming 32bit
> counters.  This causes a watchdog NMI which is delayed a little (e.g. from an
> SMI), to appear as if it hadn't overflowed, and therefore be (mis)classifed as
> not a watchdog NMI.  On systems where unknown NMIs are treated as fatal, this
> results in a spurious crash.
> 
> Switch setup_p6_watchdog() to use boot_cpu_data.cpuid_level, which is how this
> is checked almost everywhere else.
> 
> core2_vpmu_init() used the same pattern to look at leaf 0xa.  Despite being
> init code and only running on the BSP, {boot,current}_cpu_data are different
> objects, so switch it over to checking boot_cpu_data.cpuid_level too.
> 
> Fixes: 7126b7f806d5 ("x86/CPU: re-work populating of cpu_data[]")
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

> I hate this fix, but it's the only thing which I consider remotely safe to
> backport.  Recent attempts to alter CPUID ordering have 0 success at being
> bug-free.

How that? Collecting CPUID output should be doable almost first thing. There
are no (or in case of doubt: there should not be any) dependencies on about
anything else. Of course re-collecting may still be necessary after ucode
loading. Yet from what you say I must be missing something crucial.

With that in mind, I'm also questioning the Fixes: tag (without this being a
request to drop or change it): Using another CPU's data isn't much better
than using partially unset data. Unless we assumed full symmetry, at which
point re-obtaining of most data on the APs would be entirely useless. Hence
the issue was pre-existing, with one bug there hiding the issue addressed
here.

Jan