linux/Documentation/kvm/msr.txt

KVM-specific MSRs.
Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010
=====================================================

KVM makes use of some custom MSRs to service some requests.
At present, this facility is only used by kvmclock.

Custom MSRs have a range reserved for them, that goes from
0x4b564d00 to 0x4b564dff. There are MSRs outside this area,
but they are deprecated and their use is discouraged.

Custom MSR list
--------

The current supported Custom MSR list is:

MSR_KVM_WALL_CLOCK_NEW:   0x4b564d00

	data: 4-byte alignment physical address of a memory area which must be
	in guest RAM. This memory is expected to hold a copy of the following
	structure:

	struct pvclock_wall_clock {
		u32   version;
		u32   sec;
		u32   nsec;
	} __attribute__((__packed__));

	whose data will be filled in by the hypervisor. The hypervisor is only
	guaranteed to update this data at the moment of MSR write.
	Users that want to reliably query this information more than once have
	to write more than once to this MSR. Fields have the following meanings:

		version: guest has to check version before and after grabbing
		time information and check that they are both equal and even.
		An odd version indicates an in-progress update.

		sec: number of seconds for wallclock.

		nsec: number of nanoseconds for wallclock.

	Note that although MSRs are per-CPU entities, the effect of this
	particular MSR is global.

	Availability of this MSR must be checked via bit 3 in 0x4000001 cpuid
	leaf prior to usage.

MSR_KVM_SYSTEM_TIME_NEW:  0x4b564d01

	data: 4-byte aligned physical address of a memory area which must be in
	guest RAM, plus an enable bit in bit 0. This memory is expected to hold
	a copy of the following structure:

	struct pvclock_vcpu_time_info {
		u32   version;
		u32   pad0;
		u64   tsc_timestamp;
		u64   system_time;
		u32   tsc_to_system_mul;
		s8    tsc_shift;
		u8    flags;
		u8    pad[2];
	} __attribute__((__packed__)); /* 32 bytes */

	whose data will be filled in by the hypervisor periodically. Only one
	write, or registration, is needed for each VCPU. The interval between
	updates of this structure is arbitrary and implementation-dependent.
	The hypervisor may update this structure at any time it sees fit until
	anything with bit0 == 0 is written to it.

	Fields have the following meanings:

		version: guest has to check version before and after grabbing
		time information and check that they are both equal and even.
		An odd version indicates an in-progress update.

		tsc_timestamp: the tsc value at the current VCPU at the time
		of the update of this structure. Guests can subtract this value
		from current tsc to derive a notion of elapsed time since the
		structure update.

		system_time: a host notion of monotonic time, including sleep
		time at the time this structure was last updated. Unit is
		nanoseconds.

		tsc_to_system_mul: a function of the tsc frequency. One has
		to multiply any tsc-related quantity by this value to get
		a value in nanoseconds, besides dividing by 2^tsc_shift

		tsc_shift: cycle to nanosecond divider, as a power of two, to
		allow for shift rights. One has to shift right any tsc-related
		quantity by this value to get a value in nanoseconds, besides
		multiplying by tsc_to_system_mul.

		With this information, guests can derive per-CPU time by
		doing:

			time = (current_tsc - tsc_timestamp)
			time = (time * tsc_to_system_mul) >> tsc_shift
			time = time + system_time

		flags: bits in this field indicate extended capabilities
		coordinated between the guest and the hypervisor. Availability
		of specific flags has to be checked in 0x40000001 cpuid leaf.
		Current flags are:

		 flag bit   | cpuid bit    | meaning
		-------------------------------------------------------------
			    |	           | time measures taken across
		     0      |	   24      | multiple cpus are guaranteed to
			    |		   | be monotonic
		-------------------------------------------------------------

	Availability of this MSR must be checked via bit 3 in 0x4000001 cpuid
	leaf prior to usage.


MSR_KVM_WALL_CLOCK:  0x11

	data and functioning: same as MSR_KVM_WALL_CLOCK_NEW. Use that instead.

	This MSR falls outside the reserved KVM range and may be removed in the
	future. Its usage is deprecated.

	Availability of this MSR must be checked via bit 0 in 0x4000001 cpuid
	leaf prior to usage.

MSR_KVM_SYSTEM_TIME: 0x12

	data and functioning: same as MSR_KVM_SYSTEM_TIME_NEW. Use that instead.

	This MSR falls outside the reserved KVM range and may be removed in the
	future. Its usage is deprecated.

	Availability of this MSR must be checked via bit 0 in 0x4000001 cpuid
	leaf prior to usage.

	The suggested algorithm for detecting kvmclock presence is then:

		if (!kvm_para_available())    /* refer to cpuid.txt */
			return NON_PRESENT;

		flags = cpuid_eax(0x40000001);
		if (flags & 3) {
			msr_kvm_system_time = MSR_KVM_SYSTEM_TIME_NEW;
			msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK_NEW;
			return PRESENT;
		} else if (flags & 0) {
			msr_kvm_system_time = MSR_KVM_SYSTEM_TIME;
			msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK;
			return PRESENT;
		} else
			return NON_PRESENT;