[PATCH] KVM: Document basic API

[Avi Kivity <avi@redhat.com>]

Document the basic API corresponding to the 2.6.22 release.  Please review!

Signed-off-by: Avi Kivity <avi@redhat.com>
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+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 three classes
+
+ - System ioctls: These query and set global attributes which affect the
+   whole kvm subsystem.  In addition a system ioctl is used to create
+   virtual machines
+
+ - VM ioctls: These query and set attributes that affect an entire virtual
+   machine, for example memory layout.  In addition a VM ioctl is used to
+   create virtual cpus (vcpus).
+
+   Only run VM ioctls from the same process (address space) that was used
+   to create the VM.
+
+ - vcpu ioctls: These query and set attributes that control the operation
+   of a single virtual cpu.
+
+   Only run vcpu ioctls from the same thread that was used to create the
+   vcpu.
+
+2. File descritpors
+
+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 descripror which can be used to issue VM
+ioctls.  A KVM_CREATE_VCPU ioctl on a VM fd will create a virtual cpu
+and return a file descriptor pointing to it.  Finally, ioctls on a vcpu
+fd can be used to control the vcpu, including the important task of
+actually running guest code.
+
+In general file descriptors can be migrated among processes by means
+of fork() and the SCM_RIGHTS facility of unix domain socket.  These
+kinds of tricks are explicitly not supported by kvm.  While they will
+not cause harm to the host, their actual behavior is not guaranteed by
+the API.  The only supported use is one virtual machine per process,
+and one vcpu per thread.
+
+3. Extensions
+
+As of Linux 2.6.22, the KVM ABI has been stabilized: no backward
+incompatible change are allowed.  However, there is an extension
+facility that allows backward-compatible extensions to the API to be
+queried and used.
+
+The extension mechanism is not based on on the Linux version number.
+Instead, kvm defines extension identifiers and a facility to query
+whether a particular extension identifier is available.  If it is, a
+set of ioctls is available for application use.
+
+4. API description
+
+This section describes ioctls that can be used to control kvm guests.
+For each ioctl, the following information is provided along with a
+description:
+
+  Capability: which KVM extension provides this ioctl.  Can be 'basic',
+      which means that is will be provided by any kernel that supports
+      API version 12 (see section 4.1), or a KVM_CAP_xyz constant, which
+      means availability needs to be checked with KVM_CHECK_EXTENSION
+      (see section 4.4).
+
+  Architectures: which instruction set architectures provide this ioctl.
+      x86 includes both i386 and x86_64.
+
+  Type: system, vm, or vcpu.
+
+  Parameters: what parameters are accepted by the ioctl.
+
+  Returns: the return value.  General error numbers (EBADF, ENOMEM, EINVAL)
+      are not detailed, but errors with specific meanings are.
+
+4.1 KVM_GET_API_VERSION
+
+Capability: basic
+Architectures: all
+Type: system ioctl
+Parameters: none
+Returns: the constant KVM_API_VERSION (=12)
+
+This identifies the API version as the stable kvm API. It is not
+expected that this number will change.  However, Linux 2.6.20 and
+2.6.21 report earlier versions; these are not documented and not
+supported.  Applications should refuse to run if KVM_GET_API_VERSION
+returns a value other than 12.  If this check passes, all ioctls
+described as 'basic' will be available.
+
+4.2 KVM_CREATE_VM
+
+Capability: basic
+Architectures: all
+Type: system ioctl
+Parameters: none
+Returns: a VM fd that can be used to control the new virtual machine.
+
+The new VM has no virtual cpus and no memory.  An mmap() of a VM fd
+will access the virtual machine's physical address space; offset zero
+corresponds to guest physical address zero.  Use of mmap() on a VM fd
+is discouraged if userspace memory allocation (KVM_CAP_USER_MEMORY) is
+available.
+
+4.3 KVM_GET_MSR_INDEX_LIST
+
+Capability: basic
+Architectures: x86
+Type: system
+Parameters: struct kvm_msr_list (in/out)
+Returns: 0 on success; -1 on error
+Errors:
+  E2BIG:     the msr index list is to be to fit in the array specified by
+             the user.
+
+struct kvm_msr_list {
+	__u32 nmsrs; /* number of msrs in entries */
+	__u32 indices[0];
+};
+
+This ioctl returns the guest msrs that are supported.  The list varies
+by kvm version and host processor, but does not change otherwise.  The
+user fills in the size of the indices array in nmsrs, and in return
+kvm adjusts nmsrs to reflect the actual number of msrs and fills in
+the indices array with their numbers.
+
+4.4 KVM_CHECK_EXTENSION
+
+Capability: basic
+Architectures: all
+Type: system ioctl
+Parameters: extension identifier (KVM_CAP_*)
+Returns: 0 if unsupported; 1 (or some other positive integer) if supported
+
+The API allows the application to query about extensions to the core
+kvm API.  Userspace passes an extension identifier (an integer) and
+receives an integer that describes the extension availability.
+Generally 0 means no and 1 means yes, but some extensions may report
+additional information in the integer return value.
+
+4.5 KVM_GET_VCPU_MMAP_SIZE
+
+Capability: basic
+Architectures: all
+Type: system ioctl
+Parameters: none
+Returns: size of vcpu mmap area, in bytes
+
+The KVM_RUN ioctl (cf.) communicates with userspace via a shared
+memory region.  This ioctl returns the size of that region.  See the
+KVM_RUN documentation for details.
+
+4.6 KVM_SET_MEMORY_REGION
+
+Capability: basic
+Architectures: all
+Type: vm ioctl
+Parameters: struct kvm_memory_region (in)
+Returns: 0 on success, -1 on error
+
+struct kvm_memory_region {
+	__u32 slot;
+	__u32 flags;
+	__u64 guest_phys_addr;
+	__u64 memory_size; /* bytes */
+};
+
+/* for kvm_memory_region::flags */
+#define KVM_MEM_LOG_DIRTY_PAGES  1UL
+
+This ioctl allows the user to create or modify a guest physical memory
+slot.  When changing an existing slot, it may be moved in the guest
+physical memory space, or its flags may be modified.  It may not be
+resized.  Slots may not overlap.
+
+The flags field supports just one flag, KVM_MEM_LOG_DIRTY_PAGES, which
+instructs kvm to keep track of writes to memory within the slot.  See
+the KVM_GET_DIRTY_LOG ioctl.
+
+It is recommended to use the KVM_SET_USER_MEMORY_REGION ioctl instead
+of this API, if available.  This newer API allows placing guest memory
+at specified locations in the host address space, yielding better
+control and easy access.
+
+4.6 KVM_CREATE_VCPU
+
+Capability: basic
+Architectures: all
+Type: vm ioctl
+Parameters: vcpu id (apic id on x86)
+Returns: vcpu fd on success, -1 on error
+
+This API adds a vcpu to a virtual machine.  The vcpu id is a small integer
+in the range [0, max_vcpus).
+
+4.7 KVM_GET_DIRTY_LOG (vm ioctl)
+
+Capability: basic
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_dirty_log (in/out)
+Returns: 0 on success, -1 on error
+
+/* for KVM_GET_DIRTY_LOG */
+struct kvm_dirty_log {
+	__u32 slot;
+	__u32 padding;
+	union {
+		void __user *dirty_bitmap; /* one bit per page */
+		__u64 padding;
+	};
+};
+
+Given a memory slot, return a bitmap containing any pages dirtied
+since the last call to this ioctl.  Bit 0 is the first page in the
+memory slot.  Ensure the entire structure is cleared to avoid padding
+issues.
+
+4.8 KVM_SET_MEMORY_ALIAS
+
+Capability: basic
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_memory_alias (in)
+Returns: 0 (success), -1 (error)
+
+struct kvm_memory_alias {
+	__u32 slot;  /* this has a different namespace than memory slots */
+	__u32 flags;
+	__u64 guest_phys_addr;
+	__u64 memory_size;
+	__u64 target_phys_addr;
+};
+
+Defines a guest physical address space region as an alias to another
+region.  Useful for aliased address, for example the VGA low memory
+window. Should not be used with userspace memory.
+
+4.9 KVM_RUN
+
+Capability: basic
+Architectures: all
+Type: vcpu ioctl
+Parameters: none
+Returns: 0 on success, -1 on error
+Errors:
+  EINTR:     an unmasked signal is pending
+
+This ioctl is used to run a guest virtual cpu.  While there are no
+explicit parameters, there is an implicit parameter block that can be
+obtained by mmap()ing the vcpu fd at offset 0, with the size given by
+KVM_GET_VCPU_MMAP_SIZE.  The parameter block is formatted as a 'struct
+kvm_run' (see below).
+
+4.10 KVM_GET_REGS
+
+Capability: basic
+Architectures: all
+Type: vcpu ioctl
+Parameters: struct kvm_regs (out)
+Returns: 0 on success, -1 on error
+
+Reads the general purpose registers from the vcpu.
+
+/* x86 */
+struct kvm_regs {
+	/* out (KVM_GET_REGS) / in (KVM_SET_REGS) */
+	__u64 rax, rbx, rcx, rdx;
+	__u64 rsi, rdi, rsp, rbp;
+	__u64 r8,  r9,  r10, r11;
+	__u64 r12, r13, r14, r15;
+	__u64 rip, rflags;
+};
+
+4.11 KVM_SET_REGS
+
+Capability: basic
+Architectures: all
+Type: vcpu ioctl
+Parameters: struct kvm_regs (in)
+Returns: 0 on success, -1 on error
+
+Writes the general purpose registers into the vcpu.
+
+See KVM_GET_REGS for the data structure.
+
+4.12 KVM_GET_SREGS
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_sregs (out)
+Returns: 0 on success, -1 on error
+
+Reads special registers from the vcpu.
+
+/* x86 */
+struct kvm_sregs {
+	struct kvm_segment cs, ds, es, fs, gs, ss;
+	struct kvm_segment tr, ldt;
+	struct kvm_dtable gdt, idt;
+	__u64 cr0, cr2, cr3, cr4, cr8;
+	__u64 efer;
+	__u64 apic_base;
+	__u64 interrupt_bitmap[(KVM_NR_INTERRUPTS + 63) / 64];
+};
+
+interrupt_bitmap is a bitmap of pending external interrupts.  At most
+one bit may be set.  This interrupt has been acknowledged by the APIC
+but not yet injected into the cpu core.
+
+4.13 KVM_SET_SREGS
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_sregs (in)
+Returns: 0 on success, -1 on error
+
+Writes special registers into the vcpu.  See KVM_GET_SREGS for the
+data structures.
+
+4.14 KVM_TRANSLATE
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_translation (in/out)
+Returns: 0 on success, -1 on error
+
+Translates a virtual address according to the vcpu's current address
+translation mode.
+
+struct kvm_translation {
+	/* in */
+	__u64 linear_address;
+
+	/* out */
+	__u64 physical_address;
+	__u8  valid;
+	__u8  writeable;
+	__u8  usermode;
+	__u8  pad[5];
+};
+
+4.15 KVM_INTERRUPT
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_interrupt (in)
+Returns: 0 on success, -1 on error
+
+Queues a hardware interrupt vector to be injected.  This is only
+useful if in-kernel local APIC is not used.
+
+/* for KVM_INTERRUPT */
+struct kvm_interrupt {
+	/* in */
+	__u32 irq;
+};
+
+Note 'irq' is an interrupt vector, not an interrupt pin or line.
+
+4.16 KVM_DEBUG_GUEST
+
+Capability: basic
+Architectures: none
+Type: vcpu ioctl
+Parameters: none)
+Returns: -1 on error
+
+Support for this has been removed.  Use KVM_SET_GUEST_DEBUG instead.
+
+4.17 KVM_GET_MSRS
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_msrs (in/out)
+Returns: 0 on success, -1 on error
+
+Reads model-specific registers from the vcpu.  Supported msr indices can
+be obtained using KVM_GET_MSR_INDEX_LIST.
+
+struct kvm_msrs {
+	__u32 nmsrs; /* number of msrs in entries */
+	__u32 pad;
+
+	struct kvm_msr_entry entries[0];
+};
+
+struct kvm_msr_entry {
+	__u32 index;
+	__u32 reserved;
+	__u64 data;
+};
+
+Application code should set the 'nmsrs' member (which indicates the
+size of the entries array) and the 'index' member of each array entry.
+kvm will fill in the 'data' member.
+
+4.18 KVM_SET_MSRS
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_msrs (in)
+Returns: 0 on success, -1 on error
+
+Writes model-specific registers to the vcpu.  See KVM_GET_MSRS for the
+data structures.
+
+Application code should set the 'nmsrs' member (which indicates the
+size of the entries array), and the 'index' and 'data' members of each
+array entry.
+
+4.19 KVM_SET_CPUID
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_cpuid (in)
+Returns: 0 on success, -1 on error
+
+Defines the vcpu responses to the cpuid instruction.  Applications
+should use the KVM_SET_CPUID2 ioctl if available.
+
+
+struct kvm_cpuid_entry {
+	__u32 function;
+	__u32 eax;
+	__u32 ebx;
+	__u32 ecx;
+	__u32 edx;
+	__u32 padding;
+};
+
+/* for KVM_SET_CPUID */
+struct kvm_cpuid {
+	__u32 nent;
+	__u32 padding;
+	struct kvm_cpuid_entry entries[0];
+};
+
+4.20 KVM_SET_SIGNAL_MASK
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_signal_mask (in)
+Returns: 0 on success, -1 on error
+
+Defines which signals are blocked during execution of KVM_RUN.  This
+signal mask temporarily overrides the threads signal mask.  Any
+unblocked signal received (except SIGKILL and SIGSTOP, which retain
+their traditional behaviour) will cause KVM_RUN to return with -EINTR.
+
+Note the signal will only be delivered if not blocked by the original
+signal mask.
+
+/* for KVM_SET_SIGNAL_MASK */
+struct kvm_signal_mask {
+	__u32 len;
+	__u8  sigset[0];
+};
+
+4.21 KVM_GET_FPU
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_fpu (out)
+Returns: 0 on success, -1 on error
+
+Reads the floating point state from the vcpu.
+
+/* for KVM_GET_FPU and KVM_SET_FPU */
+struct kvm_fpu {
+	__u8  fpr[8][16];
+	__u16 fcw;
+	__u16 fsw;
+	__u8  ftwx;  /* in fxsave format */
+	__u8  pad1;
+	__u16 last_opcode;
+	__u64 last_ip;
+	__u64 last_dp;
+	__u8  xmm[16][16];
+	__u32 mxcsr;
+	__u32 pad2;
+};
+
+4.22 KVM_SET_FPU
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_fpu (in)
+Returns: 0 on success, -1 on error
+
+Writes the floating point state to the vcpu.
+
+/* for KVM_GET_FPU and KVM_SET_FPU */
+struct kvm_fpu {
+	__u8  fpr[8][16];
+	__u16 fcw;
+	__u16 fsw;
+	__u8  ftwx;  /* in fxsave format */
+	__u8  pad1;
+	__u16 last_opcode;
+	__u64 last_ip;
+	__u64 last_dp;
+	__u8  xmm[16][16];
+	__u32 mxcsr;
+	__u32 pad2;
+};
+
+5. The kvm_run structure
+
+Application code obtains a pointer to the kvm_run structure by
+mmap()ing a vcpu fd.  From that point, application code can control
+execution by changing fields in kvm_run prior to calling the KVM_RUN
+ioctl, and obtain information about the reason KVM_RUN returned by
+looking up structure members.
+
+struct kvm_run {
+	/* in */
+	__u8 request_interrupt_window;
+
+Request that KVM_RUN return when it becomes possible to inject external
+interrupts into the guest.  Useful in conjunction with KVM_INTERRUPT.
+
+	__u8 padding1[7];
+
+	/* out */
+	__u32 exit_reason;
+
+When KVM_RUN has returned successfully (return value 0), this informs
+application code why KVM_RUN has returned.  Allowable values for this
+field are detailed below.
+
+	__u8 ready_for_interrupt_injection;
+
+If request_interrupt_window has been specified, this field indicates
+an interrupt can be injected now with KVM_INTERRUPT.
+
+	__u8 if_flag;
+
+The value of the current interrupt flag.  Only valid if in-kernel
+local APIC is not used.
+
+	__u8 padding2[2];
+
+	/* in (pre_kvm_run), out (post_kvm_run) */
+	__u64 cr8;
+
+The value of the cr8 register.  Only valid if in-kernel local APIC is
+not used.  Both input and output.
+
+	__u64 apic_base;
+
+The value of the APIC BASE msr.  Only valid if in-kernel local
+APIC is not used.  Both input and output.
+
+	union {
+		/* KVM_EXIT_UNKNOWN */
+		struct {
+			__u64 hardware_exit_reason;
+		} hw;
+
+If exit_reason is KVM_EXIT_UNKNOWN, the vcpu has exited due to unknown
+reasons.  Further architecture-specific information is available in
+hardware_exit_reason.
+
+		/* KVM_EXIT_FAIL_ENTRY */
+		struct {
+			__u64 hardware_entry_failure_reason;
+		} fail_entry;
+
+If exit_reason is KVM_EXIT_FAIL_ENTRY, the vcpu could not be run due
+to unknown reasons.  Further architecture-specific information is
+available in hardware_entry_failure_reason.
+
+		/* KVM_EXIT_EXCEPTION */
+		struct {
+			__u32 exception;
+			__u32 error_code;
+		} ex;
+
+Unused.
+
+		/* KVM_EXIT_IO */
+		struct {
+#define KVM_EXIT_IO_IN  0
+#define KVM_EXIT_IO_OUT 1
+			__u8 direction;
+			__u8 size; /* bytes */
+			__u16 port;
+			__u32 count;
+			__u64 data_offset; /* relative to kvm_run start */
+		} io;
+
+If exit_reason is KVM_EXIT_IO_IN or KVM_EXIT_IO_OUT, then the vcpu has
+executed a port I/O instruction which could not be satisfied by kvm.
+data_offset describes where the data is located (KVM_EXIT_IO_OUT) or
+where kvm expects application code to place the data for the next
+KVM_RUN invocation (KVM_EXIT_IO_IN).  Data format is a patcked array.
+
+		struct {
+			struct kvm_debug_exit_arch arch;
+		} debug;
+
+Unused.
+
+		/* KVM_EXIT_MMIO */
+		struct {
+			__u64 phys_addr;
+			__u8  data[8];
+			__u32 len;
+			__u8  is_write;
+		} mmio;
+
+If exit_reason is KVM_EXIT_MMIO or KVM_EXIT_IO_OUT, then the vcpu has
+executed a memory-mapped I/O instruction which could not be satisfied
+by kvm.  The 'data' member contains the written data if 'is_write' is
+true, and should be filled by application code otherwise.
+
+		/* KVM_EXIT_HYPERCALL */
+		struct {
+			__u64 nr;
+			__u64 args[6];
+			__u64 ret;
+			__u32 longmode;
+			__u32 pad;
+		} hypercall;
+
+Unused.
+
+		/* KVM_EXIT_TPR_ACCESS */
+		struct {
+			__u64 rip;
+			__u32 is_write;
+			__u32 pad;
+		} tpr_access;
+
+To be documented (KVM_TPR_ACCESS_REPORTING).
+
+		/* KVM_EXIT_S390_SIEIC */
+		struct {
+			__u8 icptcode;
+			__u64 mask; /* psw upper half */
+			__u64 addr; /* psw lower half */
+			__u16 ipa;
+			__u32 ipb;
+		} s390_sieic;
+
+s390 specific.
+
+		/* KVM_EXIT_S390_RESET */
+#define KVM_S390_RESET_POR       1
+#define KVM_S390_RESET_CLEAR     2
+#define KVM_S390_RESET_SUBSYSTEM 4
+#define KVM_S390_RESET_CPU_INIT  8
+#define KVM_S390_RESET_IPL       16
+		__u64 s390_reset_flags;
+
+s390 specific.
+
+		/* KVM_EXIT_DCR */
+		struct {
+			__u32 dcrn;
+			__u32 data;
+			__u8  is_write;
+		} dcr;
+
+powerpc specific.
+
+		/* Fix the size of the union. */
+		char padding[256];
+	};
+};
-- 
1.6.0.6