[PATCH 6.18.y] KVM: TDX: Stuff exit_reason with UNDEFINED on wait_for_sept_zap return

[Sasha Levin <sashal@kernel.org>]

From: Xiaoyao Li <xiaoyao.li@intel.com>

[ Upstream commit e5f1d62c59aea4f270773de1bf6e8867f5b1124c ]

Forcefully update the exit_reason to KVM's "standard" EXIT_REASON_UNDEFINED
if tdx_vcpu_run() bails without trying to run the vCPU because KVM needs to
wait for an in-progress S-EPT zap to complete.  This avoids handling the
previous exit multiple times due to a stale exit_reason.

Fixes: 4b2abc49712b ("KVM: TDX: Kick off vCPUs when SEAMCALL is busy during TD page removal")
Cc: stable@vger.kernel.org
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://patch.msgid.link/20260819094903.3060020-6-xiaoyao.li@intel.com
[sean: use EXIT_REASON_UNDEFINED, drop comment about bits 31:16]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/kvm/vmx/tdx.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index dcbfcd79791c8..02a15ad90c586 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1041,8 +1041,16 @@ 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))) {
+		/*
+		 * The vCPU never entered the guest, but this looks like a
+		 * handled exit to the caller.  Synthesize an invalid exit
+		 * reason so the previous exit's stale value isn't consumed
+		 * a second time.
+		 */
+		vt->exit_reason.full = EXIT_REASON_UNDEFINED;
 		return EXIT_FASTPATH_EXIT_HANDLED;
+	}
 
 	trace_kvm_entry(vcpu, run_flags & KVM_RUN_FORCE_IMMEDIATE_EXIT);
 
-- 
2.53.0