KVM: x86/mmu: Replace PFERR_NESTED_GUEST_PAGE with a more descriptive helper

Drop the globally visible PFERR_NESTED_GUEST_PAGE and replace it with a
more appropriately named is_write_to_guest_page_table().  The macro name
is misleading, because while all nNPT walks match PAGE|WRITE|PRESENT, the
reverse is not true.

No functional change intended.

Link: https://lore.kernel.org/r/20240831001538.336683-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/include/asm/kvm_host.h

@@ -280,10 +280,6 @@ enum x86_intercept_stage;
 #define PFERR_PRIVATE_ACCESS   BIT_ULL(49)
 #define PFERR_SYNTHETIC_MASK   (PFERR_IMPLICIT_ACCESS | PFERR_PRIVATE_ACCESS)
 
-#define PFERR_NESTED_GUEST_PAGE (PFERR_GUEST_PAGE_MASK |	\
-				 PFERR_WRITE_MASK |		\
-				 PFERR_PRESENT_MASK)
-
 /* apic attention bits */
 #define KVM_APIC_CHECK_VAPIC	0
 /*

arch/x86/kvm/mmu/mmu.c

@@ -5945,6 +5945,13 @@ void kvm_mmu_track_write(struct kvm_vcpu *vcpu, gpa_t gpa, const u8 *new,
 	write_unlock(&vcpu->kvm->mmu_lock);
 }
 
+static bool is_write_to_guest_page_table(u64 error_code)
+{
+	const u64 mask = PFERR_GUEST_PAGE_MASK | PFERR_WRITE_MASK | PFERR_PRESENT_MASK;
+
+	return (error_code & mask) == mask;
+}
+
 int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 error_code,
 		       void *insn, int insn_len)
 {
@@ -6008,7 +6015,7 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
 	 * and resume the guest.
 	 */
 	if (vcpu->arch.mmu->root_role.direct &&
-	    (error_code & PFERR_NESTED_GUEST_PAGE) == PFERR_NESTED_GUEST_PAGE) {
+	    is_write_to_guest_page_table(error_code)) {
 		kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2_or_gpa));
 		return 1;
 	}