linux-kernel - [RFC PATCH v2 10/23] KVM: TDX: Enable huge page splitting under write kvm->mmu

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Message-ID: <20250807094333.4579-1-yan.y.zhao@intel.com>
Date: Thu,  7 Aug 2025 17:43:33 +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 10/23] KVM: TDX: Enable huge page splitting under write kvm->mmu_lock

Implement the split_external_spt hook to enable huge page splitting for
TDX when kvm->mmu_lock is held for writing.

Invoke tdh_mem_range_block(), tdh_mem_track(), kicking off vCPUs,
tdh_mem_page_demote() in sequence. All operations are performed under
kvm->mmu_lock held for writing, similar to those in page removal.

Even with kvm->mmu_lock held for writing, tdh_mem_page_demote() may still
contend with tdh_vp_enter() and potentially with the guest's S-EPT entry
operations. Therefore, kick off other vCPUs and prevent tdh_vp_enter()
from being called on them to ensure success on the second attempt. Use
KVM_BUG_ON() for any other unexpected errors.

Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
---
RFC v2:
- Split out the code to handle the error TDX_INTERRUPTED_RESTARTABLE.
- Rebased to 6.16.0-rc6 (the way of defining TDX hook changes).

RFC v1:
- Split patch for exclusive mmu_lock only,
- Invoke tdx_sept_zap_private_spte() and tdx_track() for splitting.
- Handled busy error of tdh_mem_page_demote() by kicking off vCPUs.
---
 arch/x86/kvm/vmx/tdx.c | 45 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 376287a2ddf4..8a60ba5b6595 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1915,6 +1915,50 @@ static int tdx_sept_free_private_spt(struct kvm *kvm, gfn_t gfn,
 	return 0;
 }
 
+static int tdx_spte_demote_private_spte(struct kvm *kvm, gfn_t gfn,
+					enum pg_level level, struct page *page)
+{
+	int tdx_level = pg_level_to_tdx_sept_level(level);
+	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
+	gpa_t gpa = gfn_to_gpa(gfn);
+	u64 err, entry, level_state;
+
+	err = tdh_mem_page_demote(&kvm_tdx->td, gpa, tdx_level, page,
+				  &entry, &level_state);
+
+	if (unlikely(tdx_operand_busy(err))) {
+		tdx_no_vcpus_enter_start(kvm);
+		err = tdh_mem_page_demote(&kvm_tdx->td, gpa, tdx_level, page,
+					  &entry, &level_state);
+		tdx_no_vcpus_enter_stop(kvm);
+	}
+
+	if (KVM_BUG_ON(err, kvm)) {
+		pr_tdx_error_2(TDH_MEM_PAGE_DEMOTE, err, entry, level_state);
+		return -EIO;
+	}
+	return 0;
+}
+
+static int tdx_sept_split_private_spt(struct kvm *kvm, gfn_t gfn, enum pg_level level,
+				      void *private_spt)
+{
+	struct page *page = virt_to_page(private_spt);
+	int ret;
+
+	if (KVM_BUG_ON(to_kvm_tdx(kvm)->state != TD_STATE_RUNNABLE ||
+		       level != PG_LEVEL_2M, kvm))
+		return -EINVAL;
+
+	ret = tdx_sept_zap_private_spte(kvm, gfn, level, page);
+	if (ret <= 0)
+		return ret;
+
+	tdx_track(kvm);
+
+	return tdx_spte_demote_private_spte(kvm, gfn, level, page);
+}
+
 static int tdx_sept_remove_private_spte(struct kvm *kvm, gfn_t gfn,
 					enum pg_level level, kvm_pfn_t pfn)
 {
@@ -3668,5 +3712,6 @@ void __init tdx_hardware_setup(void)
 	vt_x86_ops.set_external_spte = tdx_sept_set_private_spte;
 	vt_x86_ops.free_external_spt = tdx_sept_free_private_spt;
 	vt_x86_ops.remove_external_spte = tdx_sept_remove_private_spte;
+	vt_x86_ops.split_external_spt = tdx_sept_split_private_spt;
 	vt_x86_ops.protected_apic_has_interrupt = tdx_protected_apic_has_interrupt;
 }
-- 
2.43.2