Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 879238fe authored by Stefan Fritsch's avatar Stefan Fritsch Committed by Marcelo Tosatti
Browse files

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: default avatarStefan Fritsch <sf@sfritsch.de>
Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
parent 9fc77441

Documentation/virtual/kvm/msr.txt

+20 −12
Original line number Diff line number Diff line
@@ -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