[PATCH v4 5/9] KVM: TDX: Update exit_reason on wait_for_sept_zap return

[Xiaoyao Li]

Update exit_reason to a value with bit[31:16] cleared and bit[15:0]
set to TDX_INVALID_EXIT_REASON when it needs to return early due to
wait_for_sept_zap in tdx_vcpu_run().  This avoids the stale exit_reason
of the previous Exit being consumed twice.

Fixes: 4b2abc49712b ("KVM: TDX: Kick off vCPUs when SEAMCALL is busy during TD page removal")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
---
Changes in v4:
- new patch;
---
 arch/x86/kvm/vmx/tdx.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 987092283955..014710945e8a 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1088,8 +1088,21 @@ 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.
+		 *
+		 * Make it super clear that bit[31:16] is cleared to 0 and only
+		 * basic exit reason (bit[15:0]) is set to the synthesized
+		 * invalid exit reason.
+		 */
+		vt->exit_reason.full = 0;
+		vt->exit_reason.basic = TDX_INVALID_EXIT_REASON;
 		return EXIT_FASTPATH_EXIT_HANDLED;
+	}
 
 	trace_kvm_entry(vcpu, run_flags & KVM_RUN_FORCE_IMMEDIATE_EXIT);
 
-- 
2.43.0