linux-kernel - [PATCH 14/29] KVM: TDX: Require TDP MMU, mmio caching and EPT A/D bits for TDX

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Message-ID: <20250226195529.2314580-15-pbonzini@redhat.com>
Date: Wed, 26 Feb 2025 14:55:14 -0500
From: Paolo Bonzini <pbonzini@...hat.com>
To: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc: seanjc@...gle.com,
	Yan Zhao <yan.y.zhao@...el.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Isaku Yamahata <isaku.yamahata@...el.com>
Subject: [PATCH 14/29] KVM: TDX: Require TDP MMU, mmio caching and EPT A/D bits for TDX

From: Isaku Yamahata <isaku.yamahata@...el.com>

Disable TDX support when TDP MMU or mmio caching or EPT A/D bits aren't
supported.

As TDP MMU is becoming main stream than the legacy MMU, the legacy MMU
support for TDX isn't implemented.

TDX requires KVM mmio caching. Without mmio caching, KVM will go to MMIO
emulation without installing SPTEs for MMIOs. However, TDX guest is
protected and KVM would meet errors when trying to emulate MMIOs for TDX
guest during instruction decoding. So, TDX guest relies on SPTEs being
installed for MMIOs, which are with no RWX bits and with VE suppress bit
unset, to inject VE to TDX guest. The TDX guest would then issue TDVMCALL
in the VE handler to perform instruction decoding and have host do MMIO
emulation.

TDX also relies on EPT A/D bits as EPT A/D bits have been supported in all
CPUs since Haswell. Relying on it can avoid RWX bits being masked out in
the mirror page table for prefaulted entries.

Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
Co-developed-by: Yan Zhao <yan.y.zhao@...el.com>
Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
---
Requested by Sean at [1].
[1] https://lore.kernel.org/kvm/Zva4aORxE9ljlMNe@google.com/
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/mmu/mmu.c  |  1 +
 arch/x86/kvm/vmx/main.c |  1 +
 arch/x86/kvm/vmx/tdx.c  | 10 ++++++++++
 3 files changed, 12 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 800684c3b2c9..48a7e6f32f7f 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -110,6 +110,7 @@ static bool __ro_after_init tdp_mmu_allowed;
 #ifdef CONFIG_X86_64
 bool __read_mostly tdp_mmu_enabled = true;
 module_param_named(tdp_mmu, tdp_mmu_enabled, bool, 0444);
+EXPORT_SYMBOL_GPL(tdp_mmu_enabled);
 #endif
 
 static int max_huge_page_level __read_mostly;
diff --git a/arch/x86/kvm/vmx/main.c b/arch/x86/kvm/vmx/main.c
index 8ed08c53c02f..a4cb3d6b2986 100644
--- a/arch/x86/kvm/vmx/main.c
+++ b/arch/x86/kvm/vmx/main.c
@@ -3,6 +3,7 @@
 
 #include "x86_ops.h"
 #include "vmx.h"
+#include "mmu.h"
 #include "nested.h"
 #include "pmu.h"
 #include "posted_intr.h"
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 09c4d314e6f5..a44a50db9199 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1528,6 +1528,16 @@ int __init tdx_bringup(void)
 	if (!enable_tdx)
 		return 0;
 
+	if (!enable_ept) {
+		pr_err("EPT is required for TDX\n");
+		goto success_disable_tdx;
+	}
+
+	if (!tdp_mmu_enabled || !enable_mmio_caching || !enable_ept_ad_bits) {
+		pr_err("TDP MMU and MMIO caching and EPT A/D bit is required for TDX\n");
+		goto success_disable_tdx;
+	}
+
 	if (!cpu_feature_enabled(X86_FEATURE_MOVDIR64B)) {
 		pr_err("tdx: MOVDIR64B is required for TDX\n");
 		goto success_disable_tdx;
-- 
2.43.5