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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 06/08/2026 7:57 am, Jan Beulich wrote:
> 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.

Oh, if you're still able to test, then that's even better.

~Andrew