Diffstat (limited to 'Documentation/virtual/kvm/msr.txt')
1 files changed, 53 insertions, 12 deletions
diff --git a/Documentation/virtual/kvm/msr.txt b/Documentation/virtual/kvm/msr.txt
index 96b41bd9752..6d470ae7b07 100644
@@ -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
- 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
+ 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;
+ time >>= -tsc_shift;
+ time = (time * tsc_to_system_mul) >> 32
time = time + system_time
flags: bits in this field indicate extended capabilities
@@ -223,3 +231,36 @@ MSR_KVM_STEAL_TIME: 0x4b564d03
steal: the amount of time in which this vCPU did not run, in
nanoseconds. Time during which the vcpu is idle, will not be
reported as steal time.
+ data: Bit 0 is 1 when PV end of interrupt is enabled on the vcpu; 0
+ when disabled. Bit 1 is reserved and must be zero. When PV end of
+ interrupt is enabled (bit 0 set), bits 63-2 hold a 4-byte aligned
+ physical address of a 4 byte memory area which must be in guest RAM and
+ must be zeroed.
+ The first, least significant bit of 4 byte memory location will be
+ written to by the hypervisor, typically at the time of interrupt
+ injection. Value of 1 means that guest can skip writing EOI to the apic
+ (using MSR or MMIO write); instead, it is sufficient to signal
+ EOI by clearing the bit in guest memory - this location will
+ later be polled by the hypervisor.
+ Value of 0 means that the EOI write is required.
+ It is always safe for the guest to ignore the optimization and perform
+ the APIC EOI write anyway.
+ Hypervisor is guaranteed to only modify this least
+ significant bit while in the current VCPU context, this means that
+ guest does not need to use either lock prefix or memory ordering
+ primitives to synchronise with the hypervisor.
+ However, hypervisor can set and clear this memory bit at any time:
+ therefore to make sure hypervisor does not interrupt the
+ guest and clear the least significant bit in the memory area
+ in the window between guest testing it to detect
+ whether it can skip EOI apic write and between guest
+ clearing it to signal EOI to the hypervisor,
+ guest must both read the least significant bit in the memory area and
+ clear it using a single CPU instruction, such as test and clear, or
+ compare and exchange.