KVM/x86: add comment to kvm_mmu_do_page_fault()

On a first glance it isn't obvious why calling kvm_tdp_page_fault() in
kvm_mmu_do_page_fault() is special cased, as the general case of using
an indirect case would result in calling of kvm_tdp_page_fault()
anyway.

Add a comment to explain the reason.

Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20241108161416.28552-1-jgross@suse.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: jgross1

arch/x86/kvm/mmu/mmu_internal.h

@@ -321,6 +321,10 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 		fault.slot = kvm_vcpu_gfn_to_memslot(vcpu, fault.gfn);
 	}
 
+	/*
+	 * With retpoline being active an indirect call is rather expensive,
+	 * so do a direct call in the most common case.
+	 */
 	if (IS_ENABLED(CONFIG_MITIGATION_RETPOLINE) && fault.is_tdp)
 		r = kvm_tdp_page_fault(vcpu, &fault);
 	else