Virtual Interrupt

This section introduces the ACRN guest virtual interrupt management, which includes:

  • VCPU request for virtual interrupt kick off,
  • vPIC/vIOAPIC/vLAPIC for virtual interrupt injection interfaces,
  • physical-to-virtual interrupt mapping for a pass-thru device, and
  • the process of VMX interrupt/exception injection.

A standard VM never owns any physical interrupts; all interrupts received by the Guest OS come from a virtual interrupt injected by vLAPIC, vIOAPIC, or vPIC. Such virtual interrupts are triggered either from a pass-through device or from I/O mediators in the Service VM via hypercalls. The Interrupt Remapping section discusses how the hypervisor manages the mapping between physical and virtual interrupts for pass-through devices. However, a hard RT VM with LAPIC pass-through does own the physical maskable external interrupts. On its physical CPUs, interrupts are disabled in VMX root mode, while in VMX non-root mode, physical interrupts will be delivered to RT VM directly.

Emulation for devices is inside the Service VM user space device model, i.e., acrn-dm. However, for performance consideration, vLAPIC, vIOAPIC, and vPIC are emulated inside HV directly.

From the guest OS point of view, vPIC is Virtual Wire Mode via vIOAPIC. The symmetric I/O Mode is shown in Figure 172 later in this section.

The following command line options to guest Linux affects whether it uses PIC or IOAPIC:

  • Kernel boot param with vPIC: add “maxcpu=0” Guest OS will use PIC
  • Kernel boot param with vIOAPIC: add “maxcpu=1” (as long as not “0”) Guest OS will use IOAPIC. And Keep IOAPIC pin2 as source of PIC.

vCPU Request for Interrupt Injection

The vCPU request mechanism (described in Pending Request Handlers) is leveraged to inject interrupts to a certain vCPU. As mentioned in IPI Management, physical vector 0xF0 is used to kick VCPU out of its VMX non-root mode, used to make a request for virtual interrupt injection or other requests such as flush EPT.

Note

the IPI based vCPU request mechanism doesn’t work for the hard RT VM.

The eventid supported for virtual interrupt injection includes:

ACRN_REQUEST_EXCP

Request for exception injection.

ACRN_REQUEST_EVENT

Request for vLAPIC event.

ACRN_REQUEST_EXTINT

Request for external interrupt from vPIC.

ACRN_REQUEST_NMI

Request for non-maskable interrupt.

ACRN_REQUEST_EOI_EXIT_BITMAP_UPDATE

Request for EOI exit bitmap update.

ACRN_REQUEST_EPT_FLUSH

Request for EPT flush.

ACRN_REQUEST_TRP_FAULT

Request for triple fault.

ACRN_REQUEST_VPID_FLUSH

Request for VPID TLB flush.

ACRN_REQUEST_INIT_VMCS

Request for initilizing VMCS.

The vcpu_make_request is necessary for a virtual interrupt injection. If the target vCPU is running under VMX non-root mode, it will send an IPI to kick it out, which leads to an external-interrupt VM-Exit. In some cases, there is no need to send IPI when making a request, because the CPU making the request itself is the target VCPU. For example, the #GP exception request always happens on the current CPU when it finds an invalid emulation has happened. An external interrupt for a pass-thru device always happens on the VCPUs of the VM which this device is belonged to, so after it triggers an external-interrupt VM-Exit, the current CPU is the very target VCPU.

Virtual LAPIC

LAPIC is virtualized for all Guest types: Service and User VMs. Given support by the physical processor, APICv Virtual Interrupt Delivery (VID) is enabled and will support Posted-Interrupt feature. Otherwise, it will fall back to the legacy virtual interrupt injection mode.

vLAPIC provides the same features as the native LAPIC:

  • Vector mask/unmask
  • Virtual vector injections (Level or Edge trigger mode) to vCPU
  • vIOAPIC notification of EOI processing
  • TSC Timer service
  • vLAPIC support CR8 to update TPR
  • INIT/STARTUP handling

vLAPIC APIs

APIs are invoked when an interrupt source from vLAPIC needs to inject an interrupt, for example:

  • from LVT like LAPIC timer
  • from vIOAPIC for a pass-thru device interrupt
  • from an emulated device for a MSI

These APIs will finish by making a vCPU request.

bool vlapic_inject_intr(struct acrn_vlapic *vlapic, bool guest_irq_enabled, bool injected)
void vlapic_set_intr(struct acrn_vcpu *vcpu, uint32_t vector, bool level)
int32_t vlapic_set_local_intr(struct acrn_vm *vm, uint16_t vcpu_id_arg, uint32_t lvt_index)

Triggers LAPIC local interrupt(LVT).

Preconditions
vm != NULL
Parameters
  • vm: Pointer to VM data structure
  • vcpu_id_arg: ID of vCPU, BROADCAST_CPU_ID means triggering interrupt to all vCPUs.
  • lvt_index: The index which LVT would to be fired.
Return Value
  • 0: on success.
  • -EINVAL: on error that vcpu_id_arg or vector of the LVT is invalid.

int32_t vlapic_intr_msi(struct acrn_vm *vm, uint64_t addr, uint64_t msg)

Inject MSI to target VM.

Preconditions
vm != NULL
Parameters
  • vm: Pointer to VM data structure
  • addr: MSI address.
  • msg: MSI data.
Return Value
  • 0: on success.
  • -1: on error that addr is invalid.

void vlapic_receive_intr(struct acrn_vm *vm, bool level, uint32_t dest, bool phys, uint32_t delmode, uint32_t vec, bool rh)

EOI processing

EOI virtualization is enabled if APICv virtual interrupt delivery is supported. Except for level triggered interrupts, the VM will not exit in case of EOI.

In case of no APICv virtual interrupt delivery support, vLAPIC requires EOI from Guest OS whenever a vector was acknowledged and processed by guest. vLAPIC behavior is the same as HW LAPIC. Once an EOI is received, it clears the highest priority vector in ISR, and updates PPR status. vLAPIC will send an EOI message to vIOAPIC if the TMR bit is set to indicate that is a level triggered interrupt.

LAPIC passthrough based on vLAPIC

LAPIC passthrough is supported based on vLAPIC, the guest OS first boots with vLAPIC in xAPIC mode and then switches to x2APIC mode to enable the LAPIC pass-through.

In case of LAPIC passthrough based on vLAPIC, the system will have the following characteristics.

  • IRQs received by the LAPIC can be handled by the Guest VM without vmexit
  • Guest VM always see virtual LAPIC IDs for security consideration
  • most MSRs are directly accessible from Guest VM except for XAPICID, LDR and ICR. Write operations to ICR will be trapped to avoid malicious IPIs. Read operations to XAPIC and LDR will be trapped in order to make the Guest VM always see the virtual LAPIC IDs instead of the physical ones.

Virtual IOAPIC

vIOAPIC is emulated by HV when Guest accesses MMIO GPA range: 0xFEC00000-0xFEC01000. vIOAPIC for Service VM should match to the native HW IOAPIC Pin numbers. vIOAPIC for guest VM provides 48 pins. As the vIOAPIC is always associated with vLAPIC, the virtual interrupt injection from vIOAPIC will finally trigger a request for vLAPIC event by calling vLAPIC APIs.

Supported APIs:

void vioapic_set_irqline_lock(const struct acrn_vm *vm, uint32_t vgsi, uint32_t operation)

Set vIOAPIC IRQ line status.

Preconditions
irqline < vioapic_pincount(vm)
Return
None
Parameters
  • vm: Pointer to target VM
  • vgsi: GSI for the virtual interrupt
  • operation: Action options: GSI_SET_HIGH/GSI_SET_LOW/ GSI_RAISING_PULSE/GSI_FALLING_PULSE

void vioapic_set_irqline_nolock(const struct acrn_vm *vm, uint32_t vgsi, uint32_t operation)

Set vIOAPIC IRQ line status.

Similar with vioapic_set_irqline_lock(),but would not make sure operation be done with ioapic lock.

Preconditions
irqline < vioapic_pincount(vm)
Return
None
Parameters
  • vm: Pointer to target VM
  • vgsi: GSI for the virtual interrupt
  • operation: Action options: GSI_SET_HIGH/GSI_SET_LOW/ GSI_RAISING_PULSE/GSI_FALLING_PULSE

Virtual PIC

vPIC is required for TSC calculation. Normally guest OS will boot with vIOAPIC and vPIC as the source of external interrupts. On every VM Exit, HV will check if there are any pending external PIC interrupts. vPIC APIs usage are similar to vIOAPIC.

ACRN hypervisor emulates a vPIC for each VM based on IO range 0x20~0x21, 0xa0~0xa1 and 0x4d0~0x4d1.

If an interrupt source from vPIC need to inject an interrupt, the following APIs need be called, which will finally make a request for ACRN_REQUEST_EXTINT or ACRN_REQUEST_EVENT:

void vpic_set_irqline(struct acrn_vpic *vpic, uint32_t vgsi, uint32_t operation)

Set vPIC IRQ line status.

Return
None
Parameters
  • vpic: Pointer to target VM’s vpic table
  • vgsi: GSI for the virtual interrupt
  • operation: action options:GSI_SET_HIGH/GSI_SET_LOW/ GSI_RAISING_PULSE/GSI_FALLING_PULSE

The following APIs are used to query the vector needed to be injected and ACK the service (means move the interrupt from request service - IRR to in service - ISR):

void vpic_pending_intr(struct acrn_vpic *vpic, uint32_t *vecptr)

Get pending virtual interrupts for vPIC.

Return
None
Parameters
  • vpic: Pointer to target VM’s vpic table
  • vecptr: Pointer to vector buffer and will be filled with eligible vector if any.

void vpic_intr_accepted(struct acrn_vpic *vpic, uint32_t vector)

Accept virtual interrupt for vPIC.

Return
None
Preconditions
vm != NULL
Parameters
  • vpic: Pointer to target VM’s vpic table
  • vector: Target virtual interrupt vector

Virtual Exception

When doing emulation, an exception may need to be triggered in hypervisor, for example:

  • if guest accesses an invalid vMSR register,
  • hypervisor needs to inject a #GP, or
  • hypervisor needs to inject #PF when an instruction accesses a non-exist page from rip_gva during instruction emulation.

ACRN hypervisor implements virtual exception injection using these APIs:

int32_t vcpu_queue_exception(struct acrn_vcpu *vcpu, uint32_t vector_arg, uint32_t err_code_arg)

Queue exception to guest.

This exception may be injected immediately or later, depends on the exeception class.

Preconditions
vcpu != NULL
Parameters
  • vcpu: Pointer to vCPU.
  • vector_arg: Vector of the exeception.
  • err_code_arg: Error Code to be injected.
Return Value
  • 0: on success
  • -EINVAL: on error that vector is invalid.

void vcpu_inject_gp(struct acrn_vcpu *vcpu, uint32_t err_code)

Inject general protection exeception(GP) to guest.

Return
None
Preconditions
vcpu != NULL
Parameters
  • vcpu: Pointer to vCPU.
  • err_code: Error Code to be injected.

void vcpu_inject_pf(struct acrn_vcpu *vcpu, uint64_t addr, uint32_t err_code)

Inject page fault exeception(PF) to guest.

Return
None
Preconditions
vcpu != NULL
Parameters
  • vcpu: Pointer to vCPU.
  • addr: Address that result in PF.
  • err_code: Error Code to be injected.

void vcpu_inject_ud(struct acrn_vcpu *vcpu)

Inject invalid opcode exeception(UD) to guest.

Return
None
Preconditions
vcpu != NULL
Parameters
  • vcpu: Pointer to vCPU.

void vcpu_inject_ss(struct acrn_vcpu *vcpu)

Inject stack fault exeception(SS) to guest.

Return
None
Preconditions
vcpu != NULL
Parameters
  • vcpu: Pointer to vCPU.

ACRN hypervisor uses the vcpu_inject_gp/vcpu_inject_pf functions to queue exception request, and follows SDM vol3 - 6.15, Table 6-5 to generate double fault if the condition is met.

ACRN hypervisor could inject extint/nmi using the similar vcpu APIs:

void vcpu_inject_extint(struct acrn_vcpu *vcpu)

Inject external interrupt to guest.

Return
None
Preconditions
vcpu != NULL
Parameters
  • vcpu: Pointer to vCPU.

void vcpu_inject_nmi(struct acrn_vcpu *vcpu)

Inject NMI to guest.

Return
None
Preconditions
vcpu != NULL
Parameters
  • vcpu: Pointer to vCPU.

Virtual Interrupt Injection

The source of virtual interrupts comes from either DM or assigned devices.

  • For Service VM assigned devices: as most devices are assigned to the Service VM directly. Whenever there is a physical interrupt from an assigned device, the corresponding virtual interrupt will be injected to the Service VM via vLAPIC/vIOAPIC. See Device assignment.
  • For User VM assigned devices: only PCI devices could be assigned to User VM. For the standard VM and soft RT VM, the virtual interrupt injection follows the same way as Service VM. A virtual interrupt injection operation is triggered when a device’s physical interrupt occurs. For the hard RT VM, the physical interrupts are delivered to VM directly without causing VM-exit.
  • For User VM emulated devices: DM is responsible for the emulated devices’ interrupt lifecycle management. DM knows when an emulated device needs to assert a virtual IOPAIC/PIC Pin or needs to send a virtual MSI vector to Guest. These logic is entirely handled by DM. For the hard RT VM, there should be no emulated devices.
../../_images/virtint-image64.png

Figure 172 Handle pending virtual interrupt

Before APICv virtual interrupt delivery, a virtual interrupt can be injected only if guest interrupt is allowed. There are many cases that Guest RFLAGS.IF gets cleared and it would not accept any further interrupts. HV will check for the available Guest IRQ windows before injection.

NMI is unmaskable interrupt and its injection is always allowed regardless of the guest IRQ window status. If current IRQ window is not present, HV would enable MSR_IA32_VMX_PROCBASED_CTLS_IRQ_WIN (PROCBASED_CTRL.bit[2]) and VM Enter directly. The injection will be done on next VM Exit once Guest issues STI (GuestRFLAG.IF=1).

Data structures and interfaces

There is no data structure exported to the other components in the hypervisor for virtual interrupts. The APIs listed in the previous sections are meant to be called whenever a virtual interrupt should be injected or acknowledged.