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Diffstat (limited to 'Documentation/virtual/kvm/msr.txt')
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diff --git a/Documentation/virtual/kvm/msr.txt b/Documentation/virtual/kvm/msr.txt new file mode 100644 index 00000000..d079aed2 --- /dev/null +++ b/Documentation/virtual/kvm/msr.txt @@ -0,0 +1,187 @@ +KVM-specific MSRs. +Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010 +===================================================== + +KVM makes use of some custom MSRs to service some requests. + +Custom MSRs have a range reserved for them, that goes from +0x4b564d00 to 0x4b564dff. There are MSRs outside this area, +but they are deprecated and their use is discouraged. + +Custom MSR list +-------- + +The current supported Custom MSR list is: + +MSR_KVM_WALL_CLOCK_NEW: 0x4b564d00 + + data: 4-byte alignment physical address of a memory area which must be + in guest RAM. This memory is expected to hold a copy of the following + structure: + + struct pvclock_wall_clock { + u32 version; + u32 sec; + u32 nsec; + } __attribute__((__packed__)); + + whose data will be filled in by the hypervisor. The hypervisor is only + guaranteed to update this data at the moment of MSR write. + Users that want to reliably query this information more than once have + to write more than once to this MSR. Fields have the following meanings: + + version: guest has to check version before and after grabbing + 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. + + nsec: number of nanoseconds for wallclock. + + Note that although MSRs are per-CPU entities, the effect of this + particular MSR is global. + + Availability of this MSR must be checked via bit 3 in 0x4000001 cpuid + leaf prior to usage. + +MSR_KVM_SYSTEM_TIME_NEW: 0x4b564d01 + + data: 4-byte aligned physical address of a memory area which must be in + guest RAM, plus an enable bit in bit 0. This memory is expected to hold + a copy of the following structure: + + struct pvclock_vcpu_time_info { + u32 version; + u32 pad0; + u64 tsc_timestamp; + u64 system_time; + u32 tsc_to_system_mul; + s8 tsc_shift; + u8 flags; + u8 pad[2]; + } __attribute__((__packed__)); /* 32 bytes */ + + whose data will be filled in by the hypervisor periodically. Only one + write, or registration, is needed for each VCPU. The interval between + updates of this structure is arbitrary and implementation-dependent. + The hypervisor may update this structure at any time it sees fit until + anything with bit0 == 0 is written to it. + + Fields have the following meanings: + + version: guest has to check version before and after grabbing + time information and check that they are both equal and even. + An odd version indicates an in-progress update. + + tsc_timestamp: the tsc value at the current VCPU at the time + of the update of this structure. Guests can subtract this value + from current tsc to derive a notion of elapsed time since the + structure update. + + system_time: a host notion of monotonic time, including sleep + 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_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. + + With this information, guests can derive per-CPU time by + doing: + + time = (current_tsc - tsc_timestamp) + time = (time * tsc_to_system_mul) >> tsc_shift + time = time + system_time + + flags: bits in this field indicate extended capabilities + coordinated between the guest and the hypervisor. Availability + of specific flags has to be checked in 0x40000001 cpuid leaf. + Current flags are: + + flag bit | cpuid bit | meaning + ------------------------------------------------------------- + | | time measures taken across + 0 | 24 | multiple cpus are guaranteed to + | | be monotonic + ------------------------------------------------------------- + + Availability of this MSR must be checked via bit 3 in 0x4000001 cpuid + leaf prior to usage. + + +MSR_KVM_WALL_CLOCK: 0x11 + + data and functioning: same as MSR_KVM_WALL_CLOCK_NEW. Use that instead. + + This MSR falls outside the reserved KVM range and may be removed in the + future. Its usage is deprecated. + + Availability of this MSR must be checked via bit 0 in 0x4000001 cpuid + leaf prior to usage. + +MSR_KVM_SYSTEM_TIME: 0x12 + + data and functioning: same as MSR_KVM_SYSTEM_TIME_NEW. Use that instead. + + This MSR falls outside the reserved KVM range and may be removed in the + future. Its usage is deprecated. + + Availability of this MSR must be checked via bit 0 in 0x4000001 cpuid + leaf prior to usage. + + The suggested algorithm for detecting kvmclock presence is then: + + if (!kvm_para_available()) /* refer to cpuid.txt */ + return NON_PRESENT; + + flags = cpuid_eax(0x40000001); + if (flags & 3) { + msr_kvm_system_time = MSR_KVM_SYSTEM_TIME_NEW; + msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK_NEW; + return PRESENT; + } else if (flags & 0) { + msr_kvm_system_time = MSR_KVM_SYSTEM_TIME; + msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK; + return PRESENT; + } else + return NON_PRESENT; + +MSR_KVM_ASYNC_PF_EN: 0x4b564d02 + data: Bits 63-6 hold 64-byte aligned physical address of a + 64 byte memory area which must be in guest RAM and must be + zeroed. Bits 5-2 are reserved and should be zero. Bit 0 is 1 + when asynchronous page faults are enabled on the vcpu 0 when + disabled. Bit 2 is 1 if asynchronous page faults can be injected + when vcpu is in cpl == 0. + + First 4 byte of 64 byte memory location will be written to by + the hypervisor at the time of asynchronous page fault (APF) + injection to indicate type of asynchronous page fault. Value + of 1 means that the page referred to by the page fault is not + present. Value 2 means that the page is now available. Disabling + interrupt inhibits APFs. Guest must not enable interrupt + before the reason is read, or it may be overwritten by another + APF. Since APF uses the same exception vector as regular page + fault guest must reset the reason to 0 before it does + something that can generate normal page fault. If during page + fault APF reason is 0 it means that this is regular page + fault. + + During delivery of type 1 APF cr2 contains a token that will + be used to notify a guest when missing page becomes + available. When page becomes available type 2 APF is sent with + cr2 set to the token associated with the page. There is special + kind of token 0xffffffff which tells vcpu that it should wake + up all processes waiting for APFs and no individual type 2 APFs + will be sent. + + If APF is disabled while there are outstanding APFs, they will + not be delivered. + + Currently type 2 APF will be always delivered on the same vcpu as + type 1 was, but guest should not rely on that. |