KVM: x86/mmu: Bail from fast_page_fault() if SPTE is not shadow-present

mainline inclusion
from mainline-v5.13-rc1
commit ec89e643
category: feature
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I7S3VQ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ec89e643867148ab4a2a856a38717d2e89692be7



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Bail from fast_page_fault() if the SPTE is not a shadow-present SPTE.
Functionally, this is not strictly necessary as the !is_access_allowed()
check will eventually reject the fast path, but an early check on
shadow-present skips unnecessary checks and will allow a future patch to
tweak the A/D status auditing to warn if KVM attempts to query A/D bits
without first ensuring the SPTE is a shadow-present SPTE.

Note, is_shadow_present_pte() is quite expensive at this time, i.e. this
might be a net negative in the short term.  A future patch will optimize
is_shadow_present_pte() to a single AND operation and remedy the issue.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>