Re: [PATCH 4/7] KVM: TDX: Kick off vCPUs when SEAMCALL is busy during TD page removal

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On 1/13/2025 10:12 AM, Yan Zhao wrote:
[...]
+
  /* TDH.PHYMEM.PAGE.RECLAIM is allowed only when destroying the TD. */
  static int __tdx_reclaim_page(hpa_t pa)
  {
@@ -979,6 +999,14 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu, bool force_immediate_exit)
  		return EXIT_FASTPATH_NONE;
  	}
+ /*
+	 * Wait until retry of SEPT-zap-related SEAMCALL completes before
+	 * 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)))
+		return EXIT_FASTPATH_EXIT_HANDLED;
+
  	trace_kvm_entry(vcpu, force_immediate_exit);
if (pi_test_on(&tdx->pi_desc)) {
@@ -1647,15 +1675,23 @@ static int tdx_sept_drop_private_spte(struct kvm *kvm, gfn_t gfn,
  	if (KVM_BUG_ON(!is_hkid_assigned(kvm_tdx), kvm))
  		return -EINVAL;
- do {
+	/*
+	 * When zapping private page, write lock is held. So no race condition
+	 * with other vcpu sept operation.
+	 * Race with TDH.VP.ENTER due to (0-step mitigation) and Guest TDCALLs.
+	 */
+	err = tdh_mem_page_remove(kvm_tdx->tdr_pa, gpa, tdx_level, &entry,
+				  &level_state);
+	if ((err & TDX_OPERAND_BUSY)) {

It is not safe to use "err & TDX_OPERAND_BUSY".
E.g., if the error is TDX_EPT_WALK_FAILED, "err & TDX_OPERAND_BUSY" will be true.

Maybe you can add a helper to check it.

staticinlinebooltdx_operand_busy(u64err)
{
return(err &TDX_SEAMCALL_STATUS_MASK) ==TDX_OPERAND_BUSY;
}


  		/*
-		 * When zapping private page, write lock is held. So no race
-		 * condition with other vcpu sept operation.  Race only with
-		 * TDH.VP.ENTER.
+		 * The second retry is expected to succeed after kicking off all
+		 * other vCPUs and prevent them from invoking TDH.VP.ENTER.
  		 */
+		tdx_no_vcpus_enter_start(kvm);
  		err = tdh_mem_page_remove(kvm_tdx->tdr_pa, gpa, tdx_level, &entry,
  					  &level_state);
-	} while (unlikely(err == TDX_ERROR_SEPT_BUSY));
+		tdx_no_vcpus_enter_stop(kvm);
+	}
if (unlikely(kvm_tdx->state != TD_STATE_RUNNABLE &&
  		     err == (TDX_EPT_WALK_FAILED | TDX_OPERAND_ID_RCX))) {
@@ -1726,8 +1762,12 @@ static int tdx_sept_zap_private_spte(struct kvm *kvm, gfn_t gfn,
  	WARN_ON_ONCE(level != PG_LEVEL_4K);
err = tdh_mem_range_block(kvm_tdx->tdr_pa, gpa, tdx_level, &entry, &level_state);
-	if (unlikely(err == TDX_ERROR_SEPT_BUSY))
-		return -EAGAIN;
+	if (unlikely(err & TDX_OPERAND_BUSY)) {
Ditto.

+		/* After no vCPUs enter, the second retry is expected to succeed */
+		tdx_no_vcpus_enter_start(kvm);
+		err = tdh_mem_range_block(kvm_tdx->tdr_pa, gpa, tdx_level, &entry, &level_state);
+		tdx_no_vcpus_enter_stop(kvm);
+	}
  	if (KVM_BUG_ON(err, kvm)) {
  		pr_tdx_error_2(TDH_MEM_RANGE_BLOCK, err, entry, level_state);
  		return -EIO;
@@ -1770,9 +1810,13 @@ static void tdx_track(struct kvm *kvm)
lockdep_assert_held_write(&kvm->mmu_lock); - do {
+	err = tdh_mem_track(kvm_tdx->tdr_pa);
+	if ((err & TDX_SEAMCALL_STATUS_MASK) == TDX_OPERAND_BUSY) {
+		/* After no vCPUs enter, the second retry is expected to succeed */
+		tdx_no_vcpus_enter_start(kvm);
  		err = tdh_mem_track(kvm_tdx->tdr_pa);
-	} while (unlikely((err & TDX_SEAMCALL_STATUS_MASK) == TDX_OPERAND_BUSY));
+		tdx_no_vcpus_enter_stop(kvm);
+	}
[...]




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