Re: [PATCH 5/5] x86/nmi: Don't configure EvtSel repeatedly

On 05.08.2026 17:37, Andrew Cooper wrote:
> On 05/08/2026 3:20 pm, Jan Beulich wrote:
>> On 05.08.2026 14:45, Andrew Cooper wrote:
>>> In both setup_{k7,p6}_watchdog(), EvtSel0 is first zeroed, then written with
>>> everything but the enable bit, then written with the enable bit.
>>>
>>> setup_p4_watchdog() is slightly more complicated, owing to what
>>> appears to be a bug introduced by commit 2a2bd8de16b6 ("Clean up NMI
>>> watchdog handler."), which causes a second bit to be temporarily
>>> different too.
>>>
>>> The middle of the three writes is useless in all cases.  Drop it.
>> Spotting the 1st write in setup_p4_watchdog() wasn't quite as easy, as
>> MSR_P4_BPU_CCCR0 (as passed to clear_msr_range()) has nothing to do with
>> MSR_P4_IQ_CCCR0. Using unrelated MSR names there is as unhelpful as using
>> raw hex numbers.
> 
> Perf counters on the P4 are utterly insane, but our local logic really
> doesn't help matters.
> 
> Another option would be to remove P4 watchdog support, in the basis that
> we really can't test it.

Well, my Tulsa system is still alive, and the watchdog looks to be working
there.

Jan