KVM: x86/mmu: Try "unprotect for retry" iff there are indirect SPs

Try to unprotect shadow pages if and only if indirect_shadow_pages is non-
zero, i.e. iff there is at least one protected such shadow page.  Pre-
checking indirect_shadow_pages avoids taking mmu_lock for write when the
gfn is write-protected by a third party, i.e. not for KVM shadow paging,
and in the *extremely* unlikely case that a different task has already
unprotected the last shadow page.

Link: https://lore.kernel.org/r/20240831001538.336683-10-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu/mmu.c

@@ -2718,6 +2718,17 @@ bool kvm_mmu_unprotect_gfn_and_retry(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa)
 	gpa_t gpa = cr2_or_gpa;
 	bool r;
 
+	/*
+	 * Bail early if there aren't any write-protected shadow pages to avoid
+	 * unnecessarily taking mmu_lock lock, e.g. if the gfn is write-tracked
+	 * by a third party.  Reading indirect_shadow_pages without holding
+	 * mmu_lock is safe, as this is purely an optimization, i.e. a false
+	 * positive is benign, and a false negative will simply result in KVM
+	 * skipping the unprotect+retry path, which is also an optimization.
+	 */
+	if (!READ_ONCE(vcpu->kvm->arch.indirect_shadow_pages))
+		return false;
+
 	if (!vcpu->arch.mmu->root_role.direct)
 		gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2_or_gpa, NULL);