KVM: TDX: Fold tdx_mem_page_record_premap_cnt() into its sole caller

Fold tdx_mem_page_record_premap_cnt() into tdx_sept_set_private_spte() as
providing a one-off helper for effectively three lines of code is at best a
wash, and splitting the code makes the comment for smp_rmb()  _extremely_
confusing as the comment talks about reading kvm->arch.pre_fault_allowed
before kvm_tdx->state, but the immediately visible code does the exact
opposite.

Opportunistically rewrite the comments to more explicitly explain who is
checking what, as well as _why_ the ordering matters.

No functional change intended.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20251030200951.3402865-16-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/tdx.c

@@ -1605,29 +1605,6 @@ static int tdx_mem_page_aug(struct kvm *kvm, gfn_t gfn,
 	return 0;
 }
 
-/*
- * KVM_TDX_INIT_MEM_REGION calls kvm_gmem_populate() to map guest pages; the
- * callback tdx_gmem_post_populate() then maps pages into private memory.
- * through the a seamcall TDH.MEM.PAGE.ADD().  The SEAMCALL also requires the
- * private EPT structures for the page to have been built before, which is
- * done via kvm_tdp_map_page(). nr_premapped counts the number of pages that
- * were added to the EPT structures but not added with TDH.MEM.PAGE.ADD().
- * The counter has to be zero on KVM_TDX_FINALIZE_VM, to ensure that there
- * are no half-initialized shared EPT pages.
- */
-static int tdx_mem_page_record_premap_cnt(struct kvm *kvm, gfn_t gfn,
-					  enum pg_level level, kvm_pfn_t pfn)
-{
-	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
-
-	if (KVM_BUG_ON(kvm->arch.pre_fault_allowed, kvm))
-		return -EIO;
-
-	/* nr_premapped will be decreased when tdh_mem_page_add() is called. */
-	atomic64_inc(&kvm_tdx->nr_premapped);
-	return 0;
-}
-
 static int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn,
 				     enum pg_level level, u64 mirror_spte)
 {
@@ -1642,14 +1619,30 @@ static int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn,
 		     (mirror_spte & VMX_EPT_RWX_MASK) != VMX_EPT_RWX_MASK);
 
 	/*
-	 * Read 'pre_fault_allowed' before 'kvm_tdx->state'; see matching
-	 * barrier in tdx_td_finalize().
+	 * Ensure pre_fault_allowed is read by kvm_arch_vcpu_pre_fault_memory()
+	 * before kvm_tdx->state.  Userspace must not be allowed to pre-fault
+	 * arbitrary memory until the initial memory image is finalized.  Pairs
+	 * with the smp_wmb() in tdx_td_finalize().
 	 */
 	smp_rmb();
-	if (likely(kvm_tdx->state == TD_STATE_RUNNABLE))
-		return tdx_mem_page_aug(kvm, gfn, level, pfn);
 
-	return tdx_mem_page_record_premap_cnt(kvm, gfn, level, pfn);
+	/*
+	 * If the TD isn't finalized/runnable, then userspace is initializing
+	 * the VM image via KVM_TDX_INIT_MEM_REGION.  Increment the number of
+	 * pages that need to be mapped and initialized via TDH.MEM.PAGE.ADD.
+	 * KVM_TDX_FINALIZE_VM checks the counter to ensure all pre-mapped
+	 * pages have been added to the image, to prevent running the TD with a
+	 * valid mapping in the mirror EPT, but not in the S-EPT.
+	 */
+	if (unlikely(kvm_tdx->state != TD_STATE_RUNNABLE)) {
+		if (KVM_BUG_ON(kvm->arch.pre_fault_allowed, kvm))
+			return -EIO;
+
+		atomic64_inc(&kvm_tdx->nr_premapped);
+		return 0;
+	}
+
+	return tdx_mem_page_aug(kvm, gfn, level, pfn);
 }
 
 static int tdx_sept_link_private_spt(struct kvm *kvm, gfn_t gfn,