[PATCH v3 0/4] KVM: TDX: Enable VM-DoS Prevention Features for TDX

[PATCH v3 0/4] KVM: TDX: Enable VM-DoS Prevention Features for TDX

[Xiaoyao Li]

Hi all,

This is v3 of the series to enable the Notify VM Exit and Bus Lock VM
exit for TDX, which fixes the KVM CAP issue related with them and allow
userspace to actually enable the features.

It still tries to keep the series as small as possible because it's
targeted for being backported to stable kenrels.  This v3 splits the
patch2 of v2 into two patches (patch2 and patch3 in this v3) to make them
easier for review.  There are other issues of existing code found during
previous review but not mandatory for this series. The plan is to address
them in followup series separately.

Please refer to v1 for a full background.

v2: https://lore.kernel.org/all/20260810112200.2326727-1-xiaoyao.li@intel.com/
v1: https://lore.kernel.org/all/20260805031257.1844914-1-xiaoyao.li@intel.com/


Xiaoyao Li (4):
  KVM: TDX: Enable Notify VM exit
  KVM: TDX: Set bits 31:16 to 0 for the synthesized Exit Reason
  KVM: TDX: Don't assume exit_reason[31:16] as all-0 in
    tdx_to_vmx_exit_reason()
  KVM: TDX: Enable Bus Lock VM exit

 arch/x86/kvm/vmx/common.h | 19 ++++++++++
 arch/x86/kvm/vmx/tdx.c    | 75 +++++++++++++++++++++++++++++++--------
 arch/x86/kvm/vmx/vmx.c    | 15 ++------
 arch/x86/kvm/vmx/vmx.h    |  1 +
 4 files changed, 82 insertions(+), 28 deletions(-)


base-commit: 1b731e5ded480bd1e5546aed35584238661ce72e
-- 
2.43.0

[PATCH v3 1/4] KVM: TDX: Enable Notify VM exit

[Xiaoyao Li]

Enable Notify VM exit functionality for TDX guests.

Notify VM exit is an existing feature supported by KVM.  Userspace can
enable Notify VM exit through KVM_CAP_X86_NOTIFY_VMEXIT when it's
reported as supported.  However, KVM reports the support of this CAP just
based on the hardware capability but doesn't differentiate between VMX
and TDX.  This leads to the issue that userspace can enable this cap for
TDX guests without getting an error, but the feature is not actually
enabled because KVM doesn't call the TDX module API to program the
relevant TD VMCS fields.

Enable Notify VM exit for TDX guests by:

  - Invoking TDX module API calls to set NOTIFY_VM_EXITING and Notify
    Window in TD VMCS.  It's done in tdx_vcpu_init() where other TD VMCS
    bits are set.  Since TDX vCPU cannot be reset, it only needs to be
    configured once when initializing the TDX vCPU.

  - Adding corresponding exit handler for TDX Notify VM Exit.

Notify VM exit can happen when executing the IRET instruction.  If the
IRET unblocks the NMI blocking state, bit 12 of the exit qualification
is set.  In this case, the VMM needs to restore the "blocked by NMI" state
when it decides to re-enter the guest.  For TDX, KVM cannot manage the
GUEST_INTERRUPTIBILITY_INFO and it's TDX module's responsibility to
handle it.  Extract the common part without NMI blocking handling into a
helper in common.h so that it can be shared between VMX and TDX.

Note, KVM uses "pre-production" terminology for the feature formally called
Notify VM-Exit.  All public versions of the SDM refer to the feature as
Instruction Timeout.  This will be remedied in the near future, for now,
use KVM's terminology for consistency.

Note, #2, there is no enumeration bit for Notify VM exit by TDX module
because all TDX modules support it, and allow to set the corresponding
TD VMCS fields as long as the hardware supports the feature.

Fixes: 161d34609f9b ("KVM: TDX: Make TDX VM type supported")
Cc: stable@vger.kernel.org
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
Changes in v3:
- Collect R-b tag from Rick

Changes in v2:
- Mention the feature name mismatch between KVM and SDM in changelog and
  leave the renaming to future, since this patch is targeted for stable
- Extract the common handling into a helper, and put the helper in
  common.h instead of refactorin the existing handle_notify() in vmx.h
- Add a note to clarify the feature is always supported by TDX module,
  to make Sashiko happy.
---
 arch/x86/kvm/vmx/common.h | 19 +++++++++++++++++++
 arch/x86/kvm/vmx/tdx.c    | 10 ++++++++++
 arch/x86/kvm/vmx/vmx.c    | 13 +------------
 3 files changed, 30 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kvm/vmx/common.h b/arch/x86/kvm/vmx/common.h
index 08005676702c..2cbaa9aba901 100644
--- a/arch/x86/kvm/vmx/common.h
+++ b/arch/x86/kvm/vmx/common.h
@@ -4,6 +4,7 @@
 
 #include <linux/kvm_host.h>
 #include <asm/posted_intr.h>
+#include <asm/vmx.h>
 
 #include "mmu.h"
 
@@ -183,6 +184,24 @@ static inline void __vmx_deliver_posted_interrupt(struct kvm_vcpu *vcpu,
 	kvm_vcpu_trigger_posted_interrupt(vcpu, POSTED_INTR_VECTOR);
 }
 
+static inline int __vmx_handle_notify(struct kvm_vcpu *vcpu,
+				      unsigned long exit_qual)
+{
+	bool context_invalid = exit_qual & NOTIFY_VM_CONTEXT_INVALID;
+
+	++vcpu->stat.notify_window_exits;
+
+	if (vcpu->kvm->arch.notify_vmexit_flags & KVM_X86_NOTIFY_VMEXIT_USER ||
+	    context_invalid) {
+		vcpu->run->exit_reason = KVM_EXIT_NOTIFY;
+		vcpu->run->notify.flags = context_invalid ?
+					  KVM_NOTIFY_CONTEXT_INVALID : 0;
+		return 0;
+	}
+
+	return 1;
+}
+
 noinstr void vmx_handle_nmi(struct kvm_vcpu *vcpu);
 
 #endif /* __KVM_X86_VMX_COMMON_H */
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index b272c20586a7..7338ac0af693 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -2126,6 +2126,9 @@ int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
 		 * - If it's not an MSMI, no need to do anything here.
 		 */
 		return 1;
+	case EXIT_REASON_NOTIFY:
+		/* NMI blocking state is handled by TDX module */
+		return __vmx_handle_notify(vcpu, vmx_get_exit_qual(vcpu));
 	default:
 		break;
 	}
@@ -3154,6 +3157,13 @@ static int tdx_vcpu_init(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *cmd)
 	td_vmcs_write64(tdx, POSTED_INTR_DESC_ADDR, __pa(&tdx->vt.pi_desc));
 	td_vmcs_setbit32(tdx, PIN_BASED_VM_EXEC_CONTROL, PIN_BASED_POSTED_INTR);
 
+	if (kvm_notify_vmexit_enabled(vcpu->kvm)) {
+		td_vmcs_setbit32(tdx, SECONDARY_VM_EXEC_CONTROL,
+				 SECONDARY_EXEC_NOTIFY_VM_EXITING);
+		td_vmcs_write32(tdx, NOTIFY_WINDOW,
+				vcpu->kvm->arch.notify_window);
+	}
+
 	tdx->state = VCPU_TD_STATE_INITIALIZED;
 
 	return 0;
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index e3bfe6aca1a0..e53cc96002c7 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6279,9 +6279,6 @@ static int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
 static int handle_notify(struct kvm_vcpu *vcpu)
 {
 	unsigned long exit_qual = vmx_get_exit_qual(vcpu);
-	bool context_invalid = exit_qual & NOTIFY_VM_CONTEXT_INVALID;
-
-	++vcpu->stat.notify_window_exits;
 
 	/*
 	 * Notify VM exit happened while executing iret from NMI,
@@ -6291,15 +6288,7 @@ static int handle_notify(struct kvm_vcpu *vcpu)
 		vmcs_set_bits(GUEST_INTERRUPTIBILITY_INFO,
 			      GUEST_INTR_STATE_NMI);
 
-	if (vcpu->kvm->arch.notify_vmexit_flags & KVM_X86_NOTIFY_VMEXIT_USER ||
-	    context_invalid) {
-		vcpu->run->exit_reason = KVM_EXIT_NOTIFY;
-		vcpu->run->notify.flags = context_invalid ?
-					  KVM_NOTIFY_CONTEXT_INVALID : 0;
-		return 0;
-	}
-
-	return 1;
+	return __vmx_handle_notify(vcpu, exit_qual);
 }
 
 static int vmx_get_msr_imm_reg(struct kvm_vcpu *vcpu)
-- 
2.43.0

[PATCH v3 2/4] KVM: TDX: Set bits 31:16 to 0 for the synthesized Exit Reason

[Xiaoyao Li]

Set bits 31:16 to 0 instead of all-1s for KVM's synthesized Exit Reason.

KVM is going to support Bus Lock VM exit for TDX, after which bit 26 of
the Exit Reason becomes meaningful and indicates that a bus lock happened.
The existing synthesized Exit Reason, -1u, will cause a false positive in
that case.  Change the synthesized Exit Reason from -1u to U16_MAX, so that
bits 31:16 are set to 0.  This also avoids the potential issues when other
bits in 31:16 become valid in the future.

As a bonus, the check for synthesized Exit Reason in tdx_failed_vmentry()
becomes unnecessary. Drop it.

Cc: stable@vger.kernel.org
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
---
Note, the checking of (*reason != TDX_INVALID_EXIT_REASON) in
tdx_get_exit_info() can cause a false-positive when the real exit reason
is EPT_MISCONFIG. This issue is orthogonal to enabling Bus Lock VM exit and
it's not urgent since EPT_MISCONFIIG is not supposed to happen unless
current KVM code is buggy. We leave the fix for this issue to the future.

Note, #2, the checking of tdx_failed_vmentry() seems to miss the case
where a real EPT_MISCONFIG happens with failed_vmentry being set.
First, in practice, EPT_MISCONFIG cannot happen with failed_vmentry being
set. Second, even if it can, this is an pre-existing issue and the next
patch can address it.

Changes in v3:
- split from the patch 2 in v2.
- define a MARCO for the synthesized invalid Exit Reason.
---
 arch/x86/kvm/vmx/tdx.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 7338ac0af693..df23db9430f0 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -921,6 +921,9 @@ static __always_inline u32 tdcall_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 	return EXIT_REASON_TDCALL;
 }
 
+/* Synthesized invalid Exit Reason */
+#define TDX_INVALID_EXIT_REASON		U16_MAX
+
 static __always_inline u32 tdx_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_tdx *tdx = to_tdx(vcpu);
@@ -934,7 +937,12 @@ static __always_inline u32 tdx_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 	case TDX_NON_RECOVERABLE_TD_WRONG_APIC_MODE:
 		break;
 	default:
-		return -1u;
+		/*
+		 * Return the synthesized invalid Exit Reason, as the TDX
+		 * module never attempted to run the vCPU, i.e. the Exit
+		 * Reason is undefined, but this is NOT a failed VM-Enter
+		 */
+		return TDX_INVALID_EXIT_REASON;
 	}
 
 	exit_reason = tdx->vp_enter_ret;
@@ -950,7 +958,7 @@ static __always_inline u32 tdx_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 		 * Defer KVM_BUG_ON() until tdx_handle_exit() because this is in
 		 * non-instrumentable code with interrupts disabled.
 		 */
-		return -1u;
+		return TDX_INVALID_EXIT_REASON;
 	default:
 		break;
 	}
@@ -981,8 +989,7 @@ static noinstr void tdx_vcpu_enter_exit(struct kvm_vcpu *vcpu)
 
 static bool tdx_failed_vmentry(struct kvm_vcpu *vcpu)
 {
-	return vmx_get_exit_reason(vcpu).failed_vmentry &&
-	       vmx_get_exit_reason(vcpu).full != -1u;
+	return vmx_get_exit_reason(vcpu).failed_vmentry;
 }
 
 static fastpath_t tdx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
@@ -2144,7 +2151,7 @@ void tdx_get_exit_info(struct kvm_vcpu *vcpu, u32 *reason,
 	struct vcpu_tdx *tdx = to_tdx(vcpu);
 
 	*reason = tdx->vt.exit_reason.full;
-	if (*reason != -1u) {
+	if (*reason != TDX_INVALID_EXIT_REASON) {
 		*info1 = vmx_get_exit_qual(vcpu);
 		*info2 = tdx->ext_exit_qualification;
 		*intr_info = vmx_get_intr_info(vcpu);
-- 
2.43.0

[PATCH v3 3/4] KVM: TDX: Don't assume exit_reason[31:16] as all-0 in tdx_to_vmx_exit_reason()

[Xiaoyao Li]

When handling the real Exit Reason, don't assume the upper 16 bits as
all-0 in tdx_to_vmx_exit_reason(), in preparation for enabling Bus Lock
VM exit.

When Bus Lock VM exit is enabled, the bit 26 of Exit Reason becomes
valid and it can be 1 with various exit reasons. Change the logic in
tdx_to_vmx_exit_reason() to check the basic Exit Reason for correctness.
Also preserve the bit 31:16 when changing the (basic) Exit Reason, to
not lose the information in bit[31:16].

Change the return type of tdx_to_vmx_exit_reason() to
"union vmx_exit_reason" for the convenience of manipulating the basic
field.

Cc: stable@vger.kernel.org
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
---
Changes in v3:
- new patch split from patch 2 of v2.
---
 arch/x86/kvm/vmx/tdx.c | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index df23db9430f0..598b85d772e3 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -924,10 +924,10 @@ static __always_inline u32 tdcall_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 /* Synthesized invalid Exit Reason */
 #define TDX_INVALID_EXIT_REASON		U16_MAX
 
-static __always_inline u32 tdx_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
+static __always_inline union vmx_exit_reason tdx_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_tdx *tdx = to_tdx(vcpu);
-	u32 exit_reason;
+	union vmx_exit_reason exit_reason;
 
 	switch (tdx->vp_enter_ret & TDX_SEAMCALL_STATUS_MASK) {
 	case TDX_SUCCESS:
@@ -942,23 +942,27 @@ static __always_inline u32 tdx_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 		 * module never attempted to run the vCPU, i.e. the Exit
 		 * Reason is undefined, but this is NOT a failed VM-Enter
 		 */
-		return TDX_INVALID_EXIT_REASON;
+		return (union vmx_exit_reason) {
+			.basic = TDX_INVALID_EXIT_REASON,
+		};
 	}
 
-	exit_reason = tdx->vp_enter_ret;
+	exit_reason.full = (u32)tdx->vp_enter_ret;
 
-	switch (exit_reason) {
+	switch (exit_reason.basic) {
 	case EXIT_REASON_TDCALL:
 		if (tdvmcall_exit_type(vcpu))
-			return EXIT_REASON_VMCALL;
-
-		return tdcall_to_vmx_exit_reason(vcpu);
+			exit_reason.basic = EXIT_REASON_VMCALL;
+		else
+			exit_reason.basic = tdcall_to_vmx_exit_reason(vcpu);
+		break;
 	case EXIT_REASON_EPT_MISCONFIG:
 		/*
 		 * Defer KVM_BUG_ON() until tdx_handle_exit() because this is in
 		 * non-instrumentable code with interrupts disabled.
 		 */
-		return TDX_INVALID_EXIT_REASON;
+		exit_reason.basic = TDX_INVALID_EXIT_REASON;
+		break;
 	default:
 		break;
 	}
@@ -975,7 +979,7 @@ static noinstr void tdx_vcpu_enter_exit(struct kvm_vcpu *vcpu)
 
 	tdx->vp_enter_ret = tdh_vp_enter(&tdx->vp, &tdx->vp_enter_args);
 
-	vt->exit_reason.full = tdx_to_vmx_exit_reason(vcpu);
+	vt->exit_reason = tdx_to_vmx_exit_reason(vcpu);
 
 	vt->exit_qualification = tdx->vp_enter_args.rcx;
 	tdx->ext_exit_qualification = tdx->vp_enter_args.rdx;
-- 
2.43.0

[PATCH v3 4/4] KVM: TDX: Enable Bus Lock VM exit

[Xiaoyao Li]

Enable Bus Lock VM exit functionality for TDX guests.

Bus Lock VM exit is an existing feature supported by KVM.  Userspace can
enable Bus Lock VM exit through KVM_BUS_LOCK_DETECTION_EXIT when it's
reported as supported.  However, KVM reports the support of this CAP
just based on the hardware capability but doesn't differentiate between
VMX and TDX.  This leads to the issue that userspace can enable this cap
for TDX guests without getting an error, but the feature is not actually
enabled because KVM doesn't call the TDX module API to program the
relevant TD VMCS fields.

Enable Bus Lock VM exit for TDX guests by programming the
BUS_LOCK_DETECTION control in the TD VMCS and by adding the exit handler.
Clear the bus_lock_detected bit to avoid being counted multiple times if
it needs to return early for wait_for_sept_zap case in tdx_vcpu_run().
Since the wait_for_sept_zap case is expected to be rare, just do the
clearing of bus_lock_detected unconditionally.

Note, there is no enumeration bit for this feature by TDX module because
all TDX modules support it, and allow to set the TD VMCS as long as the
hardware supports the feature.

Fixes: 161d34609f9b ("KVM: TDX: Make TDX VM type supported")
Cc: stable@vger.kernel.org
Originally-by: Chenyi Qiang <chenyi.qiang@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
---
Changes in v3:
- Refine the changelog. (Rick)

Changes in v2:
- Don't overwrite the negative return value to 0. (Sashiko)
- Clear the bus_lock_detected bit when it returns early for
  wait_for_sept_zap case.
- Add a note to clarify the feature is always supported by the TDX
  module, to make Sashiko happy.
---
 arch/x86/kvm/vmx/tdx.c | 28 ++++++++++++++++++++++++++--
 arch/x86/kvm/vmx/vmx.c |  2 +-
 arch/x86/kvm/vmx/vmx.h |  1 +
 3 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 598b85d772e3..e7047657ef78 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1082,8 +1082,10 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
 	 * allowing vCPU entry to avoid contention with tdh_vp_enter() and
 	 * TDCALLs.
 	 */
-	if (unlikely(READ_ONCE(to_kvm_tdx(vcpu->kvm)->wait_for_sept_zap)))
+	if (unlikely(READ_ONCE(to_kvm_tdx(vcpu->kvm)->wait_for_sept_zap))) {
+		vt->exit_reason.bus_lock_detected = 0;
 		return EXIT_FASTPATH_EXIT_HANDLED;
+	}
 
 	trace_kvm_entry(vcpu, run_flags & KVM_RUN_FORCE_IMMEDIATE_EXIT);
 
@@ -2039,7 +2041,7 @@ int tdx_complete_emulated_msr(struct kvm_vcpu *vcpu, int err)
 }
 
 
-int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
+static int __tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
 {
 	struct vcpu_tdx *tdx = to_tdx(vcpu);
 	u64 vp_enter_ret = tdx->vp_enter_ret;
@@ -2140,6 +2142,8 @@ int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
 	case EXIT_REASON_NOTIFY:
 		/* NMI blocking state is handled by TDX module */
 		return __vmx_handle_notify(vcpu, vmx_get_exit_qual(vcpu));
+	case EXIT_REASON_BUS_LOCK:
+		return handle_bus_lock_vmexit(vcpu);
 	default:
 		break;
 	}
@@ -2149,6 +2153,22 @@ int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
 	return 0;
 }
 
+int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
+{
+	int ret = __tdx_handle_exit(vcpu, fastpath);
+
+	/* Exit to user space when bus lock was detected */
+	if (vmx_get_exit_reason(vcpu).bus_lock_detected) {
+		if (ret > 0) {
+			vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;
+			ret = 0;
+		}
+
+		vcpu->run->flags |= KVM_RUN_X86_BUS_LOCK;
+	}
+	return ret;
+}
+
 void tdx_get_exit_info(struct kvm_vcpu *vcpu, u32 *reason,
 		u64 *info1, u64 *info2, u32 *intr_info, u32 *error_code)
 {
@@ -3175,6 +3195,10 @@ static int tdx_vcpu_init(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *cmd)
 				vcpu->kvm->arch.notify_window);
 	}
 
+	if (vcpu->kvm->arch.bus_lock_detection_enabled)
+		td_vmcs_setbit32(tdx, SECONDARY_VM_EXEC_CONTROL,
+				 SECONDARY_EXEC_BUS_LOCK_DETECTION);
+
 	tdx->state = VCPU_TD_STATE_INITIALIZED;
 
 	return 0;
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index e53cc96002c7..c429db9b9205 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6265,7 +6265,7 @@ static int handle_encls(struct kvm_vcpu *vcpu)
 }
 #endif /* CONFIG_X86_SGX_KVM */
 
-static int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
+int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
 {
 	/*
 	 * Hardware may or may not set the BUS_LOCK_DETECTED flag on BUS_LOCK
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index dc8517f15bc4..8faf04c09721 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -379,6 +379,7 @@ bool __vmx_vcpu_run(struct vcpu_vmx *vmx, unsigned int flags);
 void vmx_ept_load_pdptrs(struct kvm_vcpu *vcpu);
 
 void vmx_set_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type, bool set);
+int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu);
 
 static inline void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu,
 						 u32 msr, int type)
-- 
2.43.0

Re: [PATCH v3 2/4] KVM: TDX: Set bits 31:16 to 0 for the synthesized Exit Reason

[Edgecombe, Rick P]

On Wed, 2026-08-12 at 16:02 +0800, Xiaoyao Li wrote:
> Set bits 31:16 to 0 instead of all-1s for KVM's synthesized Exit Reason.
> 
> KVM is going to support Bus Lock VM exit for TDX, after which bit 26 of
> the Exit Reason becomes meaningful and indicates that a bus lock happened.
> The existing synthesized Exit Reason, -1u, will cause a false positive in
> that case.  Change the synthesized Exit Reason from -1u to U16_MAX, so that
> bits 31:16 are set to 0.  This also avoids the potential issues when other
> bits in 31:16 become valid in the future.
> 
> As a bonus, the check for synthesized Exit Reason in tdx_failed_vmentry()
> becomes unnecessary. Drop it.
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
> ---

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

Re: [PATCH v3 4/4] KVM: TDX: Enable Bus Lock VM exit

[Edgecombe, Rick P]

On Wed, 2026-08-12 at 16:02 +0800, Xiaoyao Li wrote:
> -	if (unlikely(READ_ONCE(to_kvm_tdx(vcpu->kvm)->wait_for_sept_zap)))
> +	if (unlikely(READ_ONCE(to_kvm_tdx(vcpu->kvm)->wait_for_sept_zap))) {
> +		vt->exit_reason.bus_lock_detected = 0;
>  		return EXIT_FASTPATH_EXIT_HANDLED;
> +	}
>  

This still feels hacky to me. It at least deserves a comment I think. If you
spin another version.

Re: [PATCH v3 4/4] KVM: TDX: Enable Bus Lock VM exit

[Xiaoyao Li]

On 8/13/2026 6:58 AM, Edgecombe, Rick P wrote:
> On Wed, 2026-08-12 at 16:02 +0800, Xiaoyao Li wrote:
>> -	if (unlikely(READ_ONCE(to_kvm_tdx(vcpu->kvm)->wait_for_sept_zap)))
>> +	if (unlikely(READ_ONCE(to_kvm_tdx(vcpu->kvm)->wait_for_sept_zap))) {
>> +		vt->exit_reason.bus_lock_detected = 0;
>>   		return EXIT_FASTPATH_EXIT_HANDLED;
>> +	}
>>   
> 
> This still feels hacky to me. It at least deserves a comment I think. If you
> spin another version.

I spent more time on this today and I find there is an existing issue. 
When the previous Exit Reason is EXIT_REASON_EXTERNAL_INTERRUPT, the 
early return here can go to the path due to the stale exit_reason.

kvm_x86_call(handle_exit_irqoff)(vcpu)
   vmx_handle_exit_irqoff()
     handle_external_interrupt_irqoff()

This makes the host process the external interrupt twice, and maybe more 
times if the wait_for_sept_zap remains longer.

I think we need an separate fix to set

	vt->exit_reasons.full = TDX_INVALID_EXIT_REASON;

so that if some patch is going to consume the stale Exit Reason, it can 
be caught by TDX_INVALID_EXIT_REASON.

Re: [PATCH v3 4/4] KVM: TDX: Enable Bus Lock VM exit

[Sean Christopherson]

On Thu, Aug 13, 2026, Xiaoyao Li wrote:
> On 8/13/2026 6:58 AM, Edgecombe, Rick P wrote:
> > On Wed, 2026-08-12 at 16:02 +0800, Xiaoyao Li wrote:
> > > -	if (unlikely(READ_ONCE(to_kvm_tdx(vcpu->kvm)->wait_for_sept_zap)))
> > > +	if (unlikely(READ_ONCE(to_kvm_tdx(vcpu->kvm)->wait_for_sept_zap))) {
> > > +		vt->exit_reason.bus_lock_detected = 0;
> > >   		return EXIT_FASTPATH_EXIT_HANDLED;
> > > +	}
> > 
> > This still feels hacky to me. It at least deserves a comment I think. If you
> > spin another version.
> 
> I spent more time on this today and I find there is an existing issue. When
> the previous Exit Reason is EXIT_REASON_EXTERNAL_INTERRUPT, the early return
> here can go to the path due to the stale exit_reason.
> 
> kvm_x86_call(handle_exit_irqoff)(vcpu)
>   vmx_handle_exit_irqoff()
>     handle_external_interrupt_irqoff()
> 
> This makes the host process the external interrupt twice, and maybe more
> times if the wait_for_sept_zap remains longer.
> 
> I think we need an separate fix to set
> 
> 	vt->exit_reasons.full = TDX_INVALID_EXIT_REASON;
> 
> so that if some patch is going to consume the stale Exit Reason, it can be
> caught by TDX_INVALID_EXIT_REASON.

The more I look at this, the more I'm against shoving garbage into vt->exit_reasons.
With tdx_is_exit_reason_valid(), this is trivially easy to handle, *and* explicitly
captures the logic instead of subtly rerouting KVM away from meaningful handling.

diff --git a/arch/x86/kvm/vmx/main.c b/arch/x86/kvm/vmx/main.c
index 95d89d809c19..a1f5b5dc1fa3 100644
--- a/arch/x86/kvm/vmx/main.c
+++ b/arch/x86/kvm/vmx/main.c
@@ -157,6 +157,14 @@ static fastpath_t vt_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
        return vmx_vcpu_run(vcpu, run_flags);
 }
 
+static void vt_handle_exit_irqoff(struct kvm_vcpu *vcpu)
+{
+       if (is_td_vcpu(vcpu) && !tdx_is_exit_reason_valid(vcpu))
+               return;
+
+       vmx_handle_exit_irqoff();
+}
+
 static int vt_handle_exit(struct kvm_vcpu *vcpu,
                          enum exit_fastpath_completion fastpath)
 {
@@ -1071,7 +1079,7 @@ struct kvm_x86_ops vt_x86_ops __initdata = {
        .load_mmu_pgd = vt_op(load_mmu_pgd),
 
        .check_intercept = vmx_check_intercept,
-       .handle_exit_irqoff = vmx_handle_exit_irqoff,
+       .handle_exit_irqoff = vt_op(handle_exit_irqoff),
 
        .update_cpu_dirty_logging = vt_op(update_cpu_dirty_logging),