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NVMM Initialization

NVMM initialization is performed by the nvmm_init() function, which must be invoked by emulator software before any other NVMM function.

nvmm_init() opens the NVMM device, and expects to have the proper permissions to do so. In a default configuration, this implies being part of the "nvmm" group. If using a special configuration, emulator software should arrange to have the proper permissions before invoking nvmm_init(), and can drop them after the call has completed.

It is to be noted that nvmm_init() may perform non-re-entrant operations, and should be called only once.

NVMM Capability

The nvmm_capability structure helps emulator software identify the capabilities offered by NVMM on the host:

struct nvmm_capability {
	uint64_t version;
	uint64_t state_size;
	uint64_t max_machines;
	uint64_t max_vcpus;
	uint64_t max_ram;
	struct {
		...
	} arch;
};

For example, the max_machines field indicates the maximum number of virtual machines supported, while max_vcpus indicates the maximum number of VCPUs supported per virtual machine.

Machine Ownership

When a process creates a virtual machine via nvmm_machine_create(), it is considered the owner of this machine. No other processes than the owner can operate a virtual machine.

When an owner exits, all the virtual machines associated with it are destroyed, if they were not already destroyed by the owner itself via nvmm_machine_destroy().

Virtual machines are not inherited across fork(2) operations.

Machine Configuration

Emulator software can configure several parameters of a virtual machine by using nvmm_machine_configure(). Currently, no parameters are implemented.

VCPU Configuration

Emulator software can configure several parameters of a VCPU by using nvmm_vcpu_configure(), which can take the following operations:

#define NVMM_VCPU_CONF_CALLBACKS	0
	...

The higher fields depend on the architecture.

Guest-Host Mappings

Each virtual machine has an associated guest physical memory. Emulator software is allowed to modify this guest physical memory by mapping it into some parts of its virtual address space.

Emulator software should follow the following steps to achieve that:

• Call nvmm_hva_map() to create in the host's virtual address space an area of memory that can be shared with a guest. Typically, the hva parameter will be a pointer to an area that was previously mapped via mmap(). nvmm_hva_map() will replace the content of the area, and will make it read-write (but not executable).
• Make available in the guest an area of guest physical memory, by calling nvmm_gpa_map() and passing in the hva parameter the value that was previously given to nvmm_hva_map(). nvmm_gpa_map() does not replace the content of any memory, it only creates a direct link from gpa into hva. nvmm_gpa_unmap() removes this link without modifying hva.

The guest will then be able to use the guest physical address passed in the gpa parameter of nvmm_gpa_map(). Each change the guest makes in gpa will be reflected in the host's hva, and vice versa.

It is illegal for emulator software to use munmap() on an area that was mapped via nvmm_hva_map().

VCPU State Area

A VCPU state area is a structure that entirely defines the content of the registers of a VCPU. Only one such structure exists, for x86:

struct nvmm_x64_state {
	struct nvmm_x64_state_seg segs[NVMM_X64_NSEG];
	uint64_t gprs[NVMM_X64_NGPR];
	uint64_t crs[NVMM_X64_NCR];
	uint64_t drs[NVMM_X64_NDR];
	uint64_t msrs[NVMM_X64_NMSR];
	struct nvmm_x64_state_intr intr;
	struct fxsave fpu;
};
#define nvmm_vcpu_state nvmm_x64_state

Refer to functional examples to see precisely how to use this structure.

A VCPU state area is divided in sub-states. A flags parameter is used to set and get the VCPU state; it acts as a bitmap which indicates which sub-states to set or get.

During VM exits, a partial VCPU state area is provided in exitstate, see Exit Reasons below for details.

VCPU Programming Model

A VCPU is described by a public structure, nvmm_vcpu:

struct nvmm_vcpu {
	nvmm_cpuid_t cpuid;
	struct nvmm_vcpu_state *state;
	struct nvmm_vcpu_event *event;
	struct nvmm_vcpu_exit *exit;
};

This structure is used both publicly by emulator software and internally by libnvmm. Emulator software should not modify the pointers of this structure, because they are initialized to special values by libnvmm.

A call to nvmm_vcpu_getstate() will fetch the desired parts of the VCPU state and put them in vcpu->state. A call to nvmm_vcpu_setstate() will install in the VCPU the desired parts of vcpu->state. A call to nvmm_vcpu_inject() will inject in the VCPU the event in vcpu->event. A call to nvmm_vcpu_run() will fill vcpu->exit with the VCPU exit information.

If emulator software uses several threads, a VCPU should be associated with only one thread, and only this thread should perform VCPU modifications. Emulator software should not modify the state of a VCPU with several different threads.

Exit Reasons

The nvmm_vcpu_exit structure is used to handle VM exits:

/* Generic. */
#define NVMM_VCPU_EXIT_NONE		0x0000000000000000ULL
#define NVMM_VCPU_EXIT_INVALID		0xFFFFFFFFFFFFFFFFULL
/* x86: operations. */
#define NVMM_VCPU_EXIT_MEMORY		0x0000000000000001ULL
#define NVMM_VCPU_EXIT_IO		0x0000000000000002ULL
/* x86: changes in VCPU state. */
#define NVMM_VCPU_EXIT_SHUTDOWN		0x0000000000001000ULL
#define NVMM_VCPU_EXIT_INT_READY	0x0000000000001001ULL
#define NVMM_VCPU_EXIT_NMI_READY	0x0000000000001002ULL
#define NVMM_VCPU_EXIT_HALTED		0x0000000000001003ULL
#define NVMM_VCPU_EXIT_TPR_CHANGED	0x0000000000001004ULL
/* x86: instructions. */
#define NVMM_VCPU_EXIT_RDMSR		0x0000000000002000ULL
#define NVMM_VCPU_EXIT_WRMSR		0x0000000000002001ULL
#define NVMM_VCPU_EXIT_MONITOR		0x0000000000002002ULL
#define NVMM_VCPU_EXIT_MWAIT		0x0000000000002003ULL
#define NVMM_VCPU_EXIT_CPUID		0x0000000000002004ULL

struct nvmm_vcpu_exit {
	uint64_t reason;
	union {
		...
	} u;
	struct {
		...
	} exitstate;
};

The reason field indicates the reason of the VM exit. Additional parameters describing the exit can be present in u. exitstate contains a partial, implementation-specific VCPU state, usable as a fast-path to retrieve certain state values.

It is possible that a VM exit was caused by a reason internal to the host kernel, and that emulator software should not be concerned with. In this case, the exit reason is set to NVMM_VCPU_EXIT_NONE. This gives a chance for emulator software to halt the VM in its tracks.

Refer to functional examples to see precisely how to handle VM exits.

Event Injection

It is possible to inject an event into a VCPU. An event can be a hardware interrupt, a software interrupt, or a software exception, defined by:

#define NVMM_VCPU_EVENT_EXCP	0
#define NVMM_VCPU_EVENT_INTR	1

struct nvmm_vcpu_event {
	u_int type;
	uint8_t vector;
	union {
		struct {
			uint64_t error;
		} excp;
	} u;
};

This describes an event of type type, to be sent to vector number vector, with a possible additional error code that is implementation-specific.

It is possible that the VCPU is in a state where it cannot receive this event, if:

• the event is a hardware interrupt, and the VCPU runs with interrupts disabled, or
• the event is a non-maskable interrupt (NMI), and the VCPU is already in an in-NMI context.

Emulator software can manage interrupt and NMI window-exiting via the intr component of the VCPU state. When such window-exiting is enabled, NVMM will cause a VM exit with reason NVMM_VCPU_EXIT_INT_READY or NVMM_VCPU_EXIT_NMI_READY to indicate that the guest is now able to handle the corresponding class of interrupts.

Assist Callbacks

In order to assist emulation of certain operations, libnvmm requires emulator software to register, via nvmm_vcpu_configure(), a set of callbacks described in the following structure:

struct nvmm_assist_callbacks {
	void (*io)(struct nvmm_io *);
	void (*mem)(struct nvmm_mem *);
};

These callbacks are used by libnvmm each time nvmm_assist_io() or nvmm_assist_mem() are invoked. Emulator software that does not intend to use either of these assists can put NULL in the callbacks.

I/O Assist

When a VM exit occurs with reason NVMM_VCPU_EXIT_IO, it is necessary for emulator software to emulate the associated I/O operation. libnvmm provides an easy way for emulator software to perform that.

nvmm_assist_io() will call the registered io callback function and give it a nvmm_io structure as argument. This structure describes an I/O transaction:

struct nvmm_io {
	struct nvmm_machine *mach;
	struct nvmm_vcpu *vcpu;
	uint16_t port;
	bool in;
	size_t size;
	uint8_t *data;
};

The callback can emulate the operation using this descriptor, following two unique cases:

• The operation is an input. In this case, the callback should fill data with the desired value.
• The operation is an output. In this case, the callback should read data to retrieve the desired value.

In either case, port will indicate the I/O port, in will indicate if the operation is an input, and size will indicate the size of the access.

Mem Assist

When a VM exit occurs with reason NVMM_VCPU_EXIT_MEMORY, it is necessary for emulator software to emulate the associated memory operation. libnvmm provides an easy way for emulator software to perform that, similar to the I/O Assist.

nvmm_assist_mem() will call the registered mem callback function and give it a nvmm_mem structure as argument. This structure describes a Mem transaction:

struct nvmm_mem {
	struct nvmm_machine *mach;
	struct nvmm_vcpu *vcpu;
	gpaddr_t gpa;
	bool write;
	size_t size;
	uint8_t *data;
};

The callback can emulate the operation using this descriptor, following two unique cases:

• The operation is a read. In this case, the callback should fill data with the desired value.
• The operation is a write. In this case, the callback should read data to retrieve the desired value.

In either case, gpa will indicate the guest physical address, write will indicate if the access is a write, and size will indicate the size of the access.