linux-kernel - [RFC PATCH v2 21/23] KVM: TDX: Preallocate PAMT pages to be used in split path

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Message-ID: <20250807094604.4762-1-yan.y.zhao@intel.com>
Date: Thu,  7 Aug 2025 17:46:03 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: pbonzini@...hat.com,
	seanjc@...gle.com
Cc: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org,
	x86@...nel.org,
	rick.p.edgecombe@...el.com,
	dave.hansen@...el.com,
	kas@...nel.org,
	tabba@...gle.com,
	ackerleytng@...gle.com,
	quic_eberman@...cinc.com,
	michael.roth@....com,
	david@...hat.com,
	vannapurve@...gle.com,
	vbabka@...e.cz,
	thomas.lendacky@....com,
	pgonda@...gle.com,
	zhiquan1.li@...el.com,
	fan.du@...el.com,
	jun.miao@...el.com,
	ira.weiny@...el.com,
	isaku.yamahata@...el.com,
	xiaoyao.li@...el.com,
	binbin.wu@...ux.intel.com,
	chao.p.peng@...el.com,
	yan.y.zhao@...el.com
Subject: [RFC PATCH v2 21/23] KVM: TDX: Preallocate PAMT pages to be used in split path

From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>

Preallocate a page to be used in the split_external_spt() path.

Kernel needs one PAMT page pair for external_spt and one that provided
directly to the TDH.MEM.PAGE.DEMOTE SEAMCALL.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Co-developed-by: Yan Zhao <yan.y.zhao@...el.com>
Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
---
RFC v2:
- Pulled from
  git://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git tdx/dpamt-huge.
- Implemented the flow of topup pamt_page_cache in
  tdp_mmu_split_huge_pages_root() (Yan)
---
 arch/x86/include/asm/kvm_host.h |  2 ++
 arch/x86/kvm/mmu/mmu.c          |  1 +
 arch/x86/kvm/mmu/tdp_mmu.c      | 51 +++++++++++++++++++++++++++++++++
 3 files changed, 54 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 6b6c46c27390..508b133df903 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1591,6 +1591,8 @@ struct kvm_arch {
 #define SPLIT_DESC_CACHE_MIN_NR_OBJECTS (SPTE_ENT_PER_PAGE + 1)
 	struct kvm_mmu_memory_cache split_desc_cache;
 
+	struct kvm_mmu_memory_cache pamt_page_cache;
+
 	gfn_t gfn_direct_bits;
 
 	/*
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index f23d8fc59323..e581cee37f64 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -6848,6 +6848,7 @@ static void mmu_free_vm_memory_caches(struct kvm *kvm)
 	kvm_mmu_free_memory_cache(&kvm->arch.split_desc_cache);
 	kvm_mmu_free_memory_cache(&kvm->arch.split_page_header_cache);
 	kvm_mmu_free_memory_cache(&kvm->arch.split_shadow_page_cache);
+	kvm_mmu_free_memory_cache(&kvm->arch.pamt_page_cache);
 }
 
 void kvm_mmu_uninit_vm(struct kvm *kvm)
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index eb758aaa4374..064c4e823658 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1584,6 +1584,27 @@ static bool iter_cross_boundary(struct tdp_iter *iter, gfn_t start, gfn_t end)
 		 (iter->gfn + KVM_PAGES_PER_HPAGE(iter->level)) <= end);
 }
 
+static bool need_topup_mirror_caches(struct kvm *kvm)
+{
+	int nr = tdx_nr_pamt_pages() * 2;
+
+	return kvm_mmu_memory_cache_nr_free_objects(&kvm->arch.pamt_page_cache) < nr;
+}
+
+static int topup_mirror_caches(struct kvm *kvm)
+{
+	int r, nr;
+
+	/* One for external_spt, one for TDH.MEM.PAGE.DEMOTE */
+	nr = tdx_nr_pamt_pages() * 2;
+
+	r = kvm_mmu_topup_memory_cache(&kvm->arch.pamt_page_cache, nr);
+	if (r)
+		return r;
+
+	return 0;
+}
+
 static int tdp_mmu_split_huge_pages_root(struct kvm *kvm,
 					 struct kvm_mmu_page *root,
 					 gfn_t start, gfn_t end,
@@ -1656,6 +1677,36 @@ static int tdp_mmu_split_huge_pages_root(struct kvm *kvm,
 			continue;
 		}
 
+		if (is_mirror_sp(root) && need_topup_mirror_caches(kvm)) {
+			int r;
+
+			rcu_read_unlock();
+
+			if (shared)
+				read_unlock(&kvm->mmu_lock);
+			else
+				write_unlock(&kvm->mmu_lock);
+
+			r = topup_mirror_caches(kvm);
+
+			if (shared)
+				read_lock(&kvm->mmu_lock);
+			else
+				write_lock(&kvm->mmu_lock);
+
+			if (r) {
+				trace_kvm_mmu_split_huge_page(iter.gfn,
+							      iter.old_spte,
+							      iter.level, r);
+				return r;
+			}
+
+			rcu_read_lock();
+
+			iter.yielded = true;
+			continue;
+		}
+
 		tdp_mmu_init_child_sp(sp, &iter);
 
 		if (tdp_mmu_split_huge_page(kvm, &iter, sp, shared))
-- 
2.43.2