KVM: SEV: Prefer WBNOINVD over WBINVD for cache maintenance efficiency

AMD CPUs currently execute WBINVD in the host when unregistering SEV
guest memory or when deactivating SEV guests. Such cache maintenance is
performed to prevent data corruption, wherein the encrypted (C=1)
version of a dirty cache line might otherwise only be written back
after the memory is written in a different context (ex: C=0), yielding
corruption. However, WBINVD is performance-costly, especially because
it invalidates processor caches.

Strictly-speaking, unless the SEV ASID is being recycled (meaning the
SNP firmware requires the use of WBINVD prior to DF_FLUSH), the cache
invalidation triggered by WBINVD is unnecessary; only the writeback is
needed to prevent data corruption in remaining scenarios.

To improve performance in these scenarios, use WBNOINVD when available
instead of WBINVD. WBNOINVD still writes back all dirty lines
(preventing host data corruption by SEV guests) but does *not*
invalidate processor caches. Note that the implementation of wbnoinvd()
ensures fall back to WBINVD if WBNOINVD is unavailable.

In anticipation of forthcoming optimizations to limit the WBNOINVD only
to physical CPUs that have executed SEV guests, place the call to
wbnoinvd_on_all_cpus() in a wrapper function sev_writeback_caches().

Signed-off-by: Kevin Loughlin <kevinloughlin@google.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20250201000259.3289143-3-kevinloughlin@google.com
[sean: tweak comment regarding CLFUSH]
Cc: Francesco Lavra <francescolavra.fl@gmail.com>
Link: https://lore.kernel.org/r/20250522233733.3176144-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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