KVM: clarify kvmclock documentation

- 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>

1 files changed, 20 insertions, 12 deletions

diff --git a/Documentation/virtual/kvm/msr.txt b/Documentation/virtual/kvm/msr.txt
index 73047104858..6d470ae7b07 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