RFC: Task switch emulation fails for VM86 mode

[Wanpeng Li]

Hi all,

I found that task switch emulation fails to work for VM86 mode if
guest state is invalid. It can be reproduced by running
kvm-unit-tests/taskswitch2.flat, EPT = 0 or EPT=1,
unrestricted_guest=N, emulate_invalid_guest_state=Y.

When EPT=1, unrestricted_guest=Y, emulate_invalid_state=Y, the trace
of kvm-unit-tests/taskswitch2.flat is like below:

kvm_entry: vcpu 0
kvm_exit: reason TASK_SWITCH rip 0x4008d0 info 40000058 0
kvm_entry: vcpu 0
kvm_exit: reason EXCEPTION_NMI rip 0x0 info 0 80000306
kvm_emulate_insn: 42000:0:0f 0b (0x2)
kvm_inj_exception: #UD (0x0)
kvm_entry: vcpu 0
kvm_exit: reason TASK_SWITCH rip 0x0 info c0000050 0
kvm_entry: vcpu 0

When EPT=1, unrestricted_guest=Y, emulate_invalid_state=Y, the trace
of kvm-unit-tests/taskswitch2.flat is like below, when emulating "0f
0b(#UD)" fails, it injects another #UD and triggers a task switch in
the kvm-unit-tests/taskswitch2.flat again.

kvm_exit: reason TASK_SWITCH rip 0x0 info 40000058 0
kvm_emulate_insn: 42000:0:0f 0b (0x2)
kvm_emulate_insn: 42000:0:0f 0b (0x2) failed
kvm_inj_exception: #UD (0x0)
kvm_entry: vcpu 0
kvm_exit: reason TASK_SWITCH rip 0x0 info 40000058 0
kvm_emulate_insn: 42000:0:0f 0b (0x2)
kvm_emulate_insn: 42000:0:0f 0b (0x2) failed
kvm_inj_exception: #UD (0x0)
kvm_entry: vcpu 0
kvm_exit: reason TASK_SWITCH rip 0x0 info 40000058 0
kvm_emulate_insn: 42000:0:0f 0b (0x2)
kvm_emulate_insn: 42000:0:0f 0b (0x2) failed
kvm_inj_exception: #UD (0x0)
.....................

Then I try to add the task switch emulation in the
handle_invalid_guest_state() path, however, I will get TRIPLE FAULT.

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index f76efad..758f728 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6312,11 +6312,15 @@ static int handle_invalid_guest_state(struct
kvm_vcpu *vcpu)
     u32 cpu_exec_ctrl;
     bool intr_window_requested;
     unsigned count = 130;
+    u32 exit_reason = vmx->exit_reason;

     cpu_exec_ctrl = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
     intr_window_requested = cpu_exec_ctrl & CPU_BASED_VIRTUAL_INTR_PENDING;

     while (vmx->emulation_required && count-- != 0) {
+        if (exit_reason == EXIT_REASON_TASK_SWITCH)
+            return handle_task_switch(vcpu);
+
         if (intr_window_requested && vmx_interrupt_allowed(vcpu))
             return handle_interrupt_window(&vmx->vcpu);


kvm_exit: reason TASK_SWITCH rip 0x4008d0 info 40000058 0
kvm_entry: vcpu 0
kvm_exit: reason TASK_SWITCH rip 0x0 info 40000058 0
kvm_entry: vcpu 0
kvm_exit: reason TRIPLE_FAULT rip 0xffff info 0 0
kvm_userspace_exit: reason KVM_EXIT_SHUTDOWN (8)

Any proposal is a great appreciated. :)

Regards,
Wanpeng Li