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ioctl(vcpu_fd, KVM_RUN) asks the Linux Kernel-based Virtual Machine (KVM) to run the guest virtual CPU associated with that vCPU file descriptor. When the call returns successfully, the mapped struct kvm_run tells userspace why KVM returned and, when relevant, provides data needed to handle the event. The monitor handles that condition and calls KVM_RUN again to continue the guest.
What happens when userspace calls KVM_RUN?
KVM exposes virtual machines and their vCPUs through file descriptors and ioctls. A userspace virtual-machine monitor creates the VM and a vCPU, then uses the vCPU file descriptor to call KVM_RUN. The Linux Kernel Documentation describes it as an ioctl “used to run a guest virtual cpu.” Linux Kernel Documentation: KVM API
The call does not take an explicit ioctl argument. Instead, KVM and the monitor share a parameter-and-result area: struct kvm_run, mapped from the vCPU file descriptor at offset zero. The mapping size is obtained with KVM_GET_VCPU_MMAP_SIZE. The Linux UAPI header defines the structure used for this mapping. Linux UAPI header: kvm.h
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- The monitor prepares any applicable input fields in the mapped
struct kvm_runarea. - It calls
ioctl(vcpu_fd, KVM_RUN), and KVM runs the guest vCPU. - If KVM reports a condition requiring userspace participation, or another documented return condition occurs, the call returns.
- After a successful return, the monitor checks
exit_reasonand the corresponding exit data, then performs the required work. - The monitor calls
KVM_RUNagain when it is ready to let the guest continue.
A “VM exit” is a transition out of guest execution, but it does not necessarily mean the KVM_RUN ioctl returns to userspace. KVM handles many events internally. The userspace loop sees the returns KVM exposes through this API.
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How to read struct kvm_run
The mapped structure is both an input and output area. A monitor can set documented input fields before entering KVM. When KVM_RUN returns successfully with a return value of zero, exit_reason identifies why it returned; the structure’s union contains data for that reason. As the Linux Kernel Documentation puts it, a successful return “informs application code why KVM_RUN has returned.” Linux Kernel Documentation: KVM API
Do not confuse the ioctl’s return value with exit_reason. The return value answers whether the system call succeeded. exit_reason describes a successful KVM return. On an ioctl error, inspect errno rather than treating the contents of exit_reason as a device event.
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Examples: port I/O and MMIO exits
These examples describe the userspace interface documented by KVM; port-I/O behavior is specifically associated with x86. Actual device handling depends on the virtual machine monitor and its device model.
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| Exit | What triggered it | What userspace gets or must do | Effect of re-entering KVM_RUN |
|---|---|---|---|
KVM_EXIT_IO |
On x86, the guest performed a port-I/O operation KVM could not satisfy. | The structure reports the direction, access size, port, count, and an offset to packed data in the mapped region. For input, userspace supplies the data at the indicated location. | For this listed exit, the operation is complete and guest state is consistent only after userspace re-enters the kernel with KVM_RUN. |
KVM_EXIT_MMIO |
The guest performed a memory-mapped I/O operation KVM could not satisfy. | The exit data includes the physical address, access length, direction, and data. Userspace supplies read data when the operation requires it. | For this listed exit, the operation is complete and guest state is consistent only after userspace re-enters the kernel with KVM_RUN. |
The KVM API reference recommends KVM_EXIT_IO on x86 or KVM_EXIT_MMIO on architectures other than s390 when guest functionality must interact with host userspace. It also says KVM_EXIT_IO is significantly faster than KVM_EXIT_MMIO, without giving a benchmark figure or test conditions. Treat this as the API’s guidance, not as a universal performance result for every device model. Linux Kernel Documentation: KVM API
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Why KVM_RUN returns
A successful return with a documented exit_reason is different from an interrupted or failed ioctl. The program should classify the syscall result first, then handle a successful KVM exit or follow its own interruption and error policy.
| Outcome | How to interpret it | What to check |
|---|---|---|
Return value 0 |
The ioctl succeeded. KVM’s userspace interface reports the reason in exit_reason. |
Read exit_reason and the matching union member, then perform the required handling. |
Return value -1, with errno set to EINTR |
The call was interrupted, for example because an unmasked signal was pending. | Apply the program’s signal and retry policy; do not interpret this as a successful device exit. |
Return value -1, with another error |
The ioctl failed for another reason. | Use errno and the applicable KVM API documentation to diagnose the failure. |
The immediate_exit field is a separate way to request a prompt return. When the VM supports KVM_CAP_IMMEDIATE_EXIT, KVM checks this field as KVM_RUN starts; a nonzero value requests an immediate return with -EINTR. This is not the same mechanism as request_interrupt_window.
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Interrupt-window requests and architecture-specific fields
A monitor can set request_interrupt_window to ask KVM to return when an external interrupt can be injected. If that request was made, ready_for_interrupt_injection reports whether injection is possible. This is distinct from setting immediate_exit, which asks for a prompt return rather than reporting readiness to inject an external interrupt.
Other fields, including if_flag, cr8, and apic_base, have constraints that depend in part on whether an in-kernel local APIC is used. The structure also includes architecture-specific flags and exit details. KVM_RUN exists across architectures, but that does not make every exit type, field, or behavior identical. Use the API documentation and UAPI headers for the target architecture and kernel release when implementing a monitor.
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How userspace resumes the guest safely
For KVM_EXIT_IO, KVM_EXIT_MMIO, and several other userspace-mediated exits, handling the data in userspace is not by itself the end of the operation. The API says the operation is complete and guest state consistent only after userspace re-enters the kernel with KVM_RUN. Pending-operation state is not preserved in state visible to userspace. A monitor should therefore complete its handling and re-enter KVM as required, rather than treating the operation as already complete while deferring resumption indefinitely. Linux Kernel Documentation: KVM API
In pseudocode, the distinction looks like this:
for (;;) {
int result = ioctl(vcpu_fd, KVM_RUN);
if (result == -1) {
/* Inspect errno and apply signal/error policy. */
continue;
}
switch (run->exit_reason) {
case KVM_EXIT_IO:
/* Handle the port-I/O data described by run. */
break;
case KVM_EXIT_MMIO:
/* Handle the MMIO data described by run. */
break;
default:
/* Handle other documented exit reasons. */
break;
}
/* Re-enter KVM_RUN when the exit handling is complete. */
}This sketch illustrates the ordering, not a complete monitor: a real implementation must handle each exit reason it supports and decide how its signal and error policy affects retries or shutdown.
Which documentation to use
The Linux kernel KVM API reference is maintained and can change. Its cited version was checked on October 7, 2026; the Linux mainline UAPI header’s master branch is mutable as well. For software targeting a specific deployment, check the documentation and headers for that kernel release and architecture. KVM API reference · Linux UAPI header
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