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KVM: clarify kvmclock documentation

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- 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@sfritsch.de>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
This commit is contained in:
Stefan Fritsch
2012-09-16 12:55:40 +02:00
committed by Marcelo Tosatti
parent 9fc77441e5
commit 879238fecc
1 changed files with 20 additions and 12 deletions
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32
Documentation/virtual/kvm/msr.txt
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@@ -34,9 +34,12 @@ MSR_KVM_WALL_CLOCK_NEW: 0x4b564d00
time information and check that they are both equal and even. time information and check that they are both equal and even.
An odd version indicates an in-progress update. 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 Note that although MSRs are per-CPU entities, the effect of this
particular MSR is global. 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 time at the time this structure was last updated. Unit is
nanoseconds. nanoseconds.
tsc_to_system_mul: a function of the tsc frequency. One has tsc_to_system_mul: multiplier to be used when converting
to multiply any tsc-related quantity by this value to get tsc-related quantity to nanoseconds
a value in nanoseconds, besides dividing by 2^tsc_shift
tsc_shift: cycle to nanosecond divider, as a power of two, to tsc_shift: shift to be used when converting tsc-related
allow for shift rights. One has to shift right any tsc-related quantity to nanoseconds. This shift will ensure that
quantity by this value to get a value in nanoseconds, besides multiplication with tsc_to_system_mul does not overflow.
multiplying by tsc_to_system_mul. A positive value denotes a left shift, a negative value
a right shift.
With this information, guests can derive per-CPU time by The conversion from tsc to nanoseconds involves an additional
doing: right shift by 32 bits. With this information, guests can
derive per-CPU time by doing:
time = (current_tsc - tsc_timestamp) 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 time = time + system_time
flags: bits in this field indicate extended capabilities flags: bits in this field indicate extended capabilities