This patch adds a file that documents the usage of KVM-specific
MSRs.
[ v2: added comments from Randy ]
[ v3: added comments from Avi ]
[ v4: added information about wallclock alignment ]
Signed-off-by: Glauber Costa <[email protected]>
Reviewed-by: Randy Dunlap <[email protected]>
---
Documentation/kvm/msr.txt | 153 +++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 153 insertions(+), 0 deletions(-)
create mode 100644 Documentation/kvm/msr.txt
diff --git a/Documentation/kvm/msr.txt b/Documentation/kvm/msr.txt
new file mode 100644
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--- /dev/null
+++ b/Documentation/kvm/msr.txt
@@ -0,0 +1,153 @@
+KVM-specific MSRs.
+Glauber Costa <[email protected]>, 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 alignement 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;
--
1.6.2.2
On Tue, Jun 01, 2010 at 08:22:48AM -0400, Glauber Costa wrote:
> This patch adds a file that documents the usage of KVM-specific
> MSRs.
>
> [ v2: added comments from Randy ]
> [ v3: added comments from Avi ]
> [ v4: added information about wallclock alignment ]
>
> Signed-off-by: Glauber Costa <[email protected]>
> Reviewed-by: Randy Dunlap <[email protected]>
Applied, thanks.