KVM: x86/mmu: Document the "rules" for using host_pfn_mapping_level()
Add a comment to document how host_pfn_mapping_level() can be used safely, as the line between safe and dangerous is quite thin. E.g. if KVM were to ever support in-place promotion to create huge pages, consuming the level is safe if the caller holds mmu_lock and checks that there's an existing _leaf_ SPTE, but unsafe if the caller only checks that there's a non-leaf SPTE. Opportunistically tweak the existing comments to explicitly document why KVM needs to use READ_ONCE(). No functional change intended. Signed-off-by: Sean Christopherson <seanjc@google.com> Message-Id: <20220715232107.3775620-3-seanjc@google.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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