Hi,
I found the kvmclock documentation to be rather unhelpful. This patch
should fix it.
Cheers,
Stefan
Author: Stefan Fritsch <sf@xxxxxxxxxxx>
Date: Sun Sep 16 12:30:46 2012 +0200
kvm: Fix kvmclock documentation to match reality
- mention that system time needs to be added to wallclock time
- positive tsc_shift means left shift, not right
- mention additional 32bit right shift
Signed-off-by: Stefan Fritsch <sf@xxxxxxxxxxx>
---
Documentation/virtual/kvm/msr.txt | 32 ++++++++++++++++++++------------
1 file changed, 20 insertions(+), 12 deletions(-)
diff --git a/Documentation/virtual/kvm/msr.txt b/Documentation/virtual/kvm/msr.txt
index 7304710..6d470ae 100644
--- a/Documentation/virtual/kvm/msr.txt
+++ b/Documentation/virtual/kvm/msr.txt
@@ -34,9 +34,12 @@ MSR_KVM_WALL_CLOCK_NEW: 0x4b564d00
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.
+ sec: number of seconds for wallclock at time of boot.
- nsec: number of nanoseconds for wallclock.
+ 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.
@@ -82,20 +85,25 @@ MSR_KVM_SYSTEM_TIME_NEW: 0x4b564d01
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_to_system_mul: multiplier to be used when converting
+ tsc-related quantity to nanoseconds
- 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.
+ 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.
- With this information, guests can derive per-CPU time by
- doing:
+ 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)
- time = (time * tsc_to_system_mul) >> tsc_shift
+ 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
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