Generic vcpu interface

The virtual cpu “device” also accepts the ioctls KVM_SET_DEVICE_ATTR, KVM_GET_DEVICE_ATTR, and KVM_HAS_DEVICE_ATTR. The interface uses the same struct kvm_device_attr as other devices, but targets VCPU-wide settings and controls.

The groups and attributes per virtual cpu, if any, are architecture specific.

1. GROUP: KVM_ARM_VCPU_PMU_V3_CTRL

Architectures:ARM64

1.1. ATTRIBUTE: KVM_ARM_VCPU_PMU_V3_IRQ

Parameters:in kvm_device_attr.addr the address for PMU overflow interrupt is a pointer to an int

Returns:

-EBUSY The PMU overflow interrupt is already set
-ENXIO The overflow interrupt not set when attempting to get it
-ENODEV PMUv3 not supported
-EINVAL Invalid PMU overflow interrupt number supplied or trying to set the IRQ number without using an in-kernel irqchip.

A value describing the PMUv3 (Performance Monitor Unit v3) overflow interrupt number for this vcpu. This interrupt could be a PPI or SPI, but the interrupt type must be same for each vcpu. As a PPI, the interrupt number is the same for all vcpus, while as an SPI it must be a separate number per vcpu.

1.2 ATTRIBUTE: KVM_ARM_VCPU_PMU_V3_INIT

Parameters:no additional parameter in kvm_device_attr.addr

Returns:

-ENODEV PMUv3 not supported or GIC not initialized
-ENXIO PMUv3 not properly configured or in-kernel irqchip not configured as required prior to calling this attribute
-EBUSY PMUv3 already initialized

Request the initialization of the PMUv3. If using the PMUv3 with an in-kernel virtual GIC implementation, this must be done after initializing the in-kernel irqchip.

2. GROUP: KVM_ARM_VCPU_TIMER_CTRL

Architectures:ARM64

2.1. ATTRIBUTES: KVM_ARM_VCPU_TIMER_IRQ_VTIMER, KVM_ARM_VCPU_TIMER_IRQ_PTIMER

Parameters:in kvm_device_attr.addr the address for the timer interrupt is a pointer to an int

Returns:

-EINVAL Invalid timer interrupt number
-EBUSY One or more VCPUs has already run

A value describing the architected timer interrupt number when connected to an in-kernel virtual GIC. These must be a PPI (16 <= intid < 32). Setting the attribute overrides the default values (see below).

KVM_ARM_VCPU_TIMER_IRQ_VTIMER The EL1 virtual timer intid (default: 27)
KVM_ARM_VCPU_TIMER_IRQ_PTIMER The EL1 physical timer intid (default: 30)

Setting the same PPI for different timers will prevent the VCPUs from running. Setting the interrupt number on a VCPU configures all VCPUs created at that time to use the number provided for a given timer, overwriting any previously configured values on other VCPUs. Userspace should configure the interrupt numbers on at least one VCPU after creating all VCPUs and before running any VCPUs.

3. GROUP: KVM_ARM_VCPU_PVTIME_CTRL

Architectures:ARM64

3.1 ATTRIBUTE: KVM_ARM_VCPU_PVTIME_IPA

Parameters:64-bit base address

Returns:

-ENXIO Stolen time not implemented
-EEXIST Base address already set for this VCPU
-EINVAL Base address not 64 byte aligned

Specifies the base address of the stolen time structure for this VCPU. The base address must be 64 byte aligned and exist within a valid guest memory region. See Documentation/virt/kvm/arm/pvtime.rst for more information including the layout of the stolen time structure.

4. GROUP: KVM_VCPU_TSC_CTRL

Architectures:x86

4.1 ATTRIBUTE: KVM_VCPU_TSC_OFFSET

Parameters:64-bit unsigned TSC offset

Returns:

-EFAULT Error reading/writing the provided parameter address.
-ENXIO Attribute not supported

Specifies the guest’s TSC offset relative to the host’s TSC. The guest’s TSC is then derived by the following equation:

guest_tsc = host_tsc + KVM_VCPU_TSC_OFFSET

This attribute is useful to adjust the guest’s TSC on live migration, so that the TSC counts the time during which the VM was paused. The following describes a possible algorithm to use for this purpose.

From the source VMM process:

  1. Invoke the KVM_GET_CLOCK ioctl to record the host TSC (tsc_src), kvmclock nanoseconds (guest_src), and host CLOCK_REALTIME nanoseconds (host_src).
  2. Read the KVM_VCPU_TSC_OFFSET attribute for every vCPU to record the guest TSC offset (ofs_src[i]).
  3. Invoke the KVM_GET_TSC_KHZ ioctl to record the frequency of the guest’s TSC (freq).

From the destination VMM process:

  1. Invoke the KVM_SET_CLOCK ioctl, providing the source nanoseconds from kvmclock (guest_src) and CLOCK_REALTIME (host_src) in their respective fields. Ensure that the KVM_CLOCK_REALTIME flag is set in the provided structure.

    KVM will advance the VM’s kvmclock to account for elapsed time since recording the clock values. Note that this will cause problems in the guest (e.g., timeouts) unless CLOCK_REALTIME is synchronized between the source and destination, and a reasonably short time passes between the source pausing the VMs and the destination executing steps 4-7.

  2. Invoke the KVM_GET_CLOCK ioctl to record the host TSC (tsc_dest) and kvmclock nanoseconds (guest_dest).

  3. Adjust the guest TSC offsets for every vCPU to account for (1) time elapsed since recording state and (2) difference in TSCs between the source and destination machine:

    ofs_dst[i] = ofs_src[i] -

    (guest_src - guest_dest) * freq + (tsc_src - tsc_dest)

    (“ofs[i] + tsc - guest * freq” is the guest TSC value corresponding to a time of 0 in kvmclock. The above formula ensures that it is the same on the destination as it was on the source).

  4. Write the KVM_VCPU_TSC_OFFSET attribute for every vCPU with the respective value derived in the previous step.