path: root/Documentation/virt/kvm/msr.rst

.. SPDX-License-Identifier: GPL-2.0

=================
KVM-specific MSRs
=================

:Author: Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010

KVM makes use of some custom MSRs to service some requests.

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 at time of boot.

	nsec:
		 number of nanoseconds for wallclock at time of boot.

	In order to get the current wallclock time, the system_time from
	MSR_KVM_SYSTEM_TIME_NEW needs to be added.

	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:
		multiplier to be used when converting
		tsc-related quantity to nanoseconds

	tsc_shift:
		shift to be used when converting tsc-related
		quantity to nanoseconds. This shift will ensure that
		multiplication with tsc_to_system_mul does not overflow.
		A positive value denotes a left shift, a negative value
		a right shift.

		The conversion from tsc to nanoseconds involves an additional
		right shift by 32 bits. With this information, guests can
		derive per-CPU time by doing::

			time = (current_tsc - tsc_timestamp)
			if (tsc_shift >= 0)
				time <<= tsc_shift;
			else
				time >>= -tsc_shift;
			time = (time * tsc_to_system_mul) >> 32
			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		      |
		+-----------+--------------+----------------------------------+
		|	    |		   | guest vcpu has been paused by    |
		|    1	    |	  N/A	   | the host			      |
		|	    |		   | See 4.70 in api.txt	      |
		+-----------+--------------+----------------------------------+

	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;

MSR_KVM_ASYNC_PF_EN:
	0x4b564d02

data:
	Bits 63-6 hold 64-byte aligned physical address of a
	64 byte memory area which must be in guest RAM and must be
	zeroed. Bits 5-3 are reserved and should be zero. Bit 0 is 1
	when asynchronous page faults are enabled on the vcpu 0 when
	disabled. Bit 1 is 1 if asynchronous page faults can be injected
	when vcpu is in cpl == 0. Bit 2 is 1 if asynchronous page faults
	are delivered to L1 as #PF vmexits.  Bit 2 can be set only if
	KVM_FEATURE_ASYNC_PF_VMEXIT is present in CPUID.

	First 4 byte of 64 byte memory location will be written to by
	the hypervisor at the time of asynchronous page fault (APF)
	injection to indicate type of asynchronous page fault. Value
	of 1 means that the page referred to by the page fault is not
	present. Value 2 means that the page is now available. Disabling
	interrupt inhibits APFs. Guest must not enable interrupt
	before the reason is read, or it may be overwritten by another
	APF. Since APF uses the same exception vector as regular page
	fault guest must reset the reason to 0 before it does
	something that can generate normal page fault.  If during page
	fault APF reason is 0 it means that this is regular page
	fault.

	During delivery of type 1 APF cr2 contains a token that will
	be used to notify a guest when missing page becomes
	available. When page becomes available type 2 APF is sent with
	cr2 set to the token associated with the page. There is special
	kind of token 0xffffffff which tells vcpu that it should wake
	up all processes waiting for APFs and no individual type 2 APFs
	will be sent.

	If APF is disabled while there are outstanding APFs, they will
	not be delivered.

	Currently type 2 APF will be always delivered on the same vcpu as
	type 1 was, but guest should not rely on that.

MSR_KVM_STEAL_TIME:
	0x4b564d03

data:
	64-byte alignment 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 kvm_steal_time {
		__u64 steal;
		__u32 version;
		__u32 flags;
		__u8  preempted;
		__u8  u8_pad[3];
		__u32 pad[11];
	  }

	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. Guest is required to make sure
	this structure is initialized to zero.

	Fields have the following meanings:

	version:
		a sequence counter. In other words, guest has to check
		this field before and after grabbing time information and make
		sure they are both equal and even. An odd version indicates an
		in-progress update.

	flags:
		At this point, always zero. May be used to indicate
		changes in this structure in the future.

	steal:
		the amount of time in which this vCPU did not run, in
		nanoseconds. Time during which the vcpu is idle, will not be
		reported as steal time.

	preempted:
		indicate the vCPU who owns this struct is running or
		not. Non-zero values mean the vCPU has been preempted. Zero
		means the vCPU is not preempted. NOTE, it is always zero if the
		the hypervisor doesn't support this field.

MSR_KVM_EOI_EN:
	0x4b564d04

data:
	Bit 0 is 1 when PV end of interrupt is enabled on the vcpu; 0
	when disabled.  Bit 1 is reserved and must be zero.  When PV end of
	interrupt is enabled (bit 0 set), bits 63-2 hold a 4-byte aligned
	physical address of a 4 byte memory area which must be in guest RAM and
	must be zeroed.

	The first, least significant bit of 4 byte memory location will be
	written to by the hypervisor, typically at the time of interrupt
	injection.  Value of 1 means that guest can skip writing EOI to the apic
	(using MSR or MMIO write); instead, it is sufficient to signal
	EOI by clearing the bit in guest memory - this location will
	later be polled by the hypervisor.
	Value of 0 means that the EOI write is required.

	It is always safe for the guest to ignore the optimization and perform
	the APIC EOI write anyway.

	Hypervisor is guaranteed to only modify this least
	significant bit while in the current VCPU context, this means that
	guest does not need to use either lock prefix or memory ordering
	primitives to synchronise with the hypervisor.

	However, hypervisor can set and clear this memory bit at any time:
	therefore to make sure hypervisor does not interrupt the
	guest and clear the least significant bit in the memory area
	in the window between guest testing it to detect
	whether it can skip EOI apic write and between guest
	clearing it to signal EOI to the hypervisor,
	guest must both read the least significant bit in the memory area and
	clear it using a single CPU instruction, such as test and clear, or
	compare and exchange.

MSR_KVM_POLL_CONTROL:
	0x4b564d05

	Control host-side polling.

data:
	Bit 0 enables (1) or disables (0) host-side HLT polling logic.

	KVM guests can request the host not to poll on HLT, for example if
	they are performing polling themselves.