KVM: x86/mmu: Reword a misleading comment about checking gpte_changed()

Rewrite the comment in FNAME(fetch) to explain why KVM needs to check that
the gPTE is still fresh before continuing the shadow page walk, even if
KVM already has a linked shadow page for the gPTE in question.

No functional change intended.

Link: https://lore.kernel.org/r/20240802203900.348808-4-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu/paging_tmpl.h

@@ -695,8 +695,14 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 			return RET_PF_RETRY;
 
 		/*
-		 * Verify that the gpte in the page we've just write
-		 * protected is still there.
+		 * Verify that the gpte in the page, which is now either
+		 * write-protected or unsync, wasn't modified between the fault
+		 * and acquiring mmu_lock.  This needs to be done even when
+		 * reusing an existing shadow page to ensure the information
+		 * gathered by the walker matches the information stored in the
+		 * shadow page (which could have been modified by a different
+		 * vCPU even if the page was already linked).  Holding mmu_lock
+		 * prevents the shadow page from changing after this point.
 		 */
 		if (FNAME(gpte_changed)(vcpu, gw, it.level - 1))
 			return RET_PF_RETRY;