KVM: x86/mmu: Skip the permission_fault() check on MMIO if CR0.PG=0

Skip the MMU permission_fault() check if paging is disabled when
verifying the cached MMIO GVA is usable.  The check is unnecessary and
can theoretically get a false positive since the MMU doesn't zero out
"permissions" or "pkru_mask" when guest paging is disabled.

The obvious alternative is to zero out all the bitmasks when configuring
nonpaging MMUs, but that's unnecessary work and doesn't align with the
MMU's general approach of doing as little as possible for flows that are
supposed to be unreachable.

This is nearly a nop as the false positive is nothing more than an
insignificant performance blip, and more or less limited to string MMIO
when L1 is running with paging disabled.  KVM doesn't cache MMIO if L2 is
active with nested TDP since the "GVA" is really an L2 GPA.  If L2 is
active without nested TDP, then paging can't be disabled as neither VMX
nor SVM allows entering the guest without paging of some form.

Jumping back to L1 with paging disabled, in that case direct_map is true
and so KVM will use CR2 as a GPA; the only time it doesn't is if the
fault from the emulator doesn't match or emulator_can_use_gpa(), and that
fails only on string MMIO and other instructions with multiple memory
operands.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210713163324.627647-27-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>