@@ -7,8 +7,19 @@ The Definitive KVM (Kernel-based Virtual Machine) API Documentation
1. General description
======================
-The kvm API is a set of ioctls that are issued to control various aspects
-of a virtual machine. The ioctls belong to the following classes:
+The kvm API is centered around different kinds of file descriptors
+and ioctls that can be issued to these file descriptors. An initial
+open("/dev/kvm") obtains a handle to the kvm subsystem; this handle
+can be used to issue system ioctls. A KVM_CREATE_VM ioctl on this
+handle will create a VM file descriptor which can be used to issue VM
+ioctls. A KVM_CREATE_VCPU or KVM_CREATE_DEVICE ioctl on a VM fd will
+create a virtual cpu or device and return a file descriptor pointing to
+the new resource.
+
+In other words, the kvm API is a set of ioctls that are issued to
+different kinds of file descriptor in order to control various aspects of
+a virtual machine. Depending on the file descriptor that accepts them,
+ioctls belong to the following classes:
- System ioctls: These query and set global attributes which affect the
whole kvm subsystem. In addition a system ioctl is used to create
@@ -35,18 +46,19 @@ of a virtual machine. The ioctls belong to the following classes:
device ioctls must be issued from the same process (address space) that
was used to create the VM.
-2. File descriptors
-===================
+While most ioctls are specific to one kind of file descriptor, in some
+cases the same ioctl can belong to more than one class.
-The kvm API is centered around file descriptors. An initial
-open("/dev/kvm") obtains a handle to the kvm subsystem; this handle
-can be used to issue system ioctls. A KVM_CREATE_VM ioctl on this
-handle will create a VM file descriptor which can be used to issue VM
-ioctls. A KVM_CREATE_VCPU or KVM_CREATE_DEVICE ioctl on a VM fd will
-create a virtual cpu or device and return a file descriptor pointing to
-the new resource. Finally, ioctls on a vcpu or device fd can be used
-to control the vcpu or device. For vcpus, this includes the important
-task of actually running guest code.
+The KVM API grew over time. For this reason, KVM defines many constants
+ of the form ``KVM_CAP_*``, each corresponding to a set of functionality
+provided by one or more ioctls. Availability of these "capabilities" can
+be checked with :ref:`KVM_CHECK_EXTENSION <KVM_CHECK_EXTENSION>`. Some
+capabilities also need to be enabled for VMs or VCPUs where their
+functionality is desired (see :ref:`cap_enable` and :ref:`cap_enable_vm`).
+
+
+2. Restrictions
+===============
In general file descriptors can be migrated among processes by means
of fork() and the SCM_RIGHTS facility of unix domain socket. These
Reorganize the text to mention file descriptors as early as possible. Also mention capabilities early as they are a central part of KVM's API. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> --- Documentation/virt/kvm/api.rst | 38 ++++++++++++++++++++++------------ 1 file changed, 25 insertions(+), 13 deletions(-)