[PATCH v2 4/8] x86/cacheinfo, mtrr: Skip stop_machine rendezvous in cache_ap_online() when MTRRs match

[sashiko-bot at kernel.org]

> When a single CPU is brought online at runtime
> (cache_aps_delayed_init == false), cache_ap_online() unconditionally
> calls stop_machine_from_inactive_cpu() to rendezvous all online CPUs
> while the incoming AP runs cache_cpu_init() to disable caches
> (CR0.CD = 1, plus wbinvd() when X86_FEATURE_SELFSNOOP is not set),
> reprogram MTRRs via mtrr_generic_set_state(), and re-enable caches.
> 
> During initial boot (cache_aps_init()) and across warm CPU offline/online
> cycles, the AP's hardware MTRR MSRs have already been programmed to match
> the saved mtrr_state. Stopping all online CPUs in stop_machine (and
> flushing caches on the AP) when the AP's MTRRs already match mtrr_state
> is unnecessary.
> 
> Add mtrr_generic_state_matches() to compare the local CPU's
> MSR_MTRRdefType, variable MTRR ranges, and fixed MTRR ranges against
> [ ... ]
>   Total:     8.45 s ->  7.66 s (-9.3%)
>   cache_ap_online() (cpu511 online):
>     1,354.6 us -> 6.4 us (-99.53%)
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin at soleen.com>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261019011446.834487-1-pasha.tatashin@soleen.com?part=4