KVM: x86/mmu: make apf token non-zero to fix bug

In current async pagefault logic, when a page is ready, KVM relies on
kvm_arch_can_dequeue_async_page_present() to determine whether to deliver
a READY event to the Guest. This function test token value of struct
kvm_vcpu_pv_apf_data, which must be reset to zero by Guest kernel when a
READY event is finished by Guest. If value is zero meaning that a READY
event is done, so the KVM can deliver another.
But the kvm_arch_setup_async_pf() may produce a valid token with zero
value, which is confused with previous mention and may lead the loss of
this READY event.

This bug may cause task blocked forever in Guest:
 INFO: task stress:7532 blocked for more than 1254 seconds.
       Not tainted 5.10.0 #16
 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
 task:stress          state:D stack:    0 pid: 7532 ppid:  1409
 flags:0x00000080
 Call Trace:
  __schedule+0x1e7/0x650
  schedule+0x46/0xb0
  kvm_async_pf_task_wait_schedule+0xad/0xe0
  ? exit_to_user_mode_prepare+0x60/0x70
  __kvm_handle_async_pf+0x4f/0xb0
  ? asm_exc_page_fault+0x8/0x30
  exc_page_fault+0x6f/0x110
  ? asm_exc_page_fault+0x8/0x30
  asm_exc_page_fault+0x1e/0x30
 RIP: 0033:0x402d00
 RSP: 002b:00007ffd31912500 EFLAGS: 00010206
 RAX: 0000000000071000 RBX: ffffffffffffffff RCX: 00000000021a32b0
 RDX: 000000000007d011 RSI: 000000000007d000 RDI: 00000000021262b0
 RBP: 00000000021262b0 R08: 0000000000000003 R09: 0000000000000086
 R10: 00000000000000eb R11: 00007fefbdf2baa0 R12: 0000000000000000
 R13: 0000000000000002 R14: 000000000007d000 R15: 0000000000001000

Signed-off-by: Liang Zhang <[email protected]>
Message-Id: <[email protected]>
Cc: [email protected]
Signed-off-by: Paolo Bonzini <[email protected]>

arch/x86/kvm/mmu/mmu.c

@@ -3889,12 +3889,23 @@ static void shadow_page_table_clear_flood(struct kvm_vcpu *vcpu, gva_t addr)
 	walk_shadow_page_lockless_end(vcpu);
 }
 
+static u32 alloc_apf_token(struct kvm_vcpu *vcpu)
+{
+	/* make sure the token value is not 0 */
+	u32 id = vcpu->arch.apf.id;
+
+	if (id << 12 == 0)
+		vcpu->arch.apf.id = 1;
+
+	return (vcpu->arch.apf.id++ << 12) | vcpu->vcpu_id;
+}
+
 static bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 				    gfn_t gfn)
 {
 	struct kvm_arch_async_pf arch;
 
-	arch.token = (vcpu->arch.apf.id++ << 12) | vcpu->vcpu_id;
+	arch.token = alloc_apf_token(vcpu);
 	arch.gfn = gfn;
 	arch.direct_map = vcpu->arch.mmu->direct_map;
 	arch.cr3 = vcpu->arch.mmu->get_guest_pgd(vcpu);