[PATCH v2 1/8] x86/realmode: Remove unnecessary WBINVD from AP startup trampoline

[sashiko-bot at kernel.org]

> trampoline_start executes WBINVD in 16-bit real mode before switching to
> protected mode. This WBINVD was originally added in the 1990s for
> Sequent NUMA-Q hardware (as noted by the comment in trampoline_32.S:
> "Needed for NUMA-Q should be harmless for others"), whose cross-quad
> caches came up non-coherent prior to OS initialization. Sequent NUMA-Q
> support (CONFIG_X86_NUMAQ) was removed in commit b5660ba76b41
> ("x86, platforms: Remove NUMAQ").
> 
> Per the Intel SDM (Vol. 3A, Table 10-1, note 2) and AMD APM, the INIT
> IPI used to wake secondary CPUs preserves cache contents and leaves
> CR0.CD and CR0.NW unchanged (0, as already enabled by firmware or the
> kernel). Neither sev_es_trampoline_start nor trampoline_start64
> executes WBINVD.
> 
> On modern multi-core processors with large shared last-level caches
> (such as 504 MB shared L3 per socket on Intel Xeon 6985P-C), WBINVD is
> not harmless: each execution flushes and invalidates the entire
> coherence domain's caches and takes ~85-115 ms per CPU while disturbing
> active workloads on online CPUs.
> 
> Remove the obsolete WBINVD from trampoline_start.
> 
> 2-socket, 480-CPU Intel Xeon 6985P-C (N=479, mean of 10 runs):
>   Offline:  40.40 s -> 40.46 s
>   Online:   65.83 s -> 11.14 s (-83.1%)
>   Total:   106.23 s -> 51.60 s (-51.4%)
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin at soleen.com>

Sashiko has reviewed this patch and found no issues.

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