Timekeeping in virtual machines: Difference between revisions
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There are several ways to keep track of time in a VM, but they're either very slow (e.g. HPET) or do not work correctly if the VM is migrated (e.g. TSC).
To work around this, VMs such as QEMU/KVM provide several ways keep track of time whilst sacrificing little performance.
== KVM_HC_CLOCK_PAIRING ==
This hypercall is used to get the parameters to calculate a host's clock (KVM_CLOCK_PAIRING_WALLCLOCK for CLOCK_REALTIME).
The host copies the following structure to a physical address given by the guest:
▲<source lang="c">
struct kvm_clock_pairing {
s64 sec;
Line 20 ⟶ 15:
u32 pad[9];
};
</syntaxhighlight>
A hypercall is performed with the `vmcall` instruction. On KVM, RBX, RCX, RDX and RSI are used for arguments, RAX as the hypercall number and as the return value. No other registers are clobbered (unless explicitly noted).
For example, calling KVM_HC_CLOCK_PAIRING can be done as follows on x86_64:
▲<source lang="asm">
; rdi: physical address to copy structure to
; rsi: clock type (KVM_CLOCK_PAIRING_WALLCLOCK = 0)
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vmcall
ret
</syntaxhighlight>
== pvclock ==
pvclock is a simple protocol and the fastest way to properly track system time in a VM.
To use it, write a 64-bit 4-byte aligned physical address with bit 0 set to 1 to MSR_KVM_SYSTEM_TIME_NEW (<code>0x4b564d01</code>).
The presence of this MSR is indicated by bit 3 in EAX from leaf 0x4000001 of CPUID.
The host will write the following structure to this address:
<
struct pvclock_vcpu_time_info {
u32 version;
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u8 pad[2];
};
</syntaxhighlight>
The host will automatically update this structure when necessary (e.g. when finishing a migration).
The system time <b>in nanoseconds</b> is calculated as such:
== Hyper-V TSC page == ▼
time = rdtsc() - tsc_timestamp
▲<source lang="C">
if (tsc_shift >= 0)
time <<= tsc_shift;
else
time >>= -tsc_shift;
time = (time * tsc_to_system_mul) >> 32
time = time + system_time
</syntaxhighlight>
The version field is used to detect when the structure has been / is being updated.
If the version is odd an update is in progress and the guest must not read the other fields yet.
<syntaxhighlight lang="C">
struct ms_hyperv_tsc_page {
volatile u32 tsc_sequence;
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u64 reserved2[509];
};
</syntaxhighlight>
== See also ==
=== References ===
* [https://docs.kernel.org/virt/kvm/x86/hypercalls.html Linux KVM Hypercall]
* [https://docs.kernel.org/virt/kvm/x86/msr.html KVM-specific MSRs]
* [https://opensource.com/article/17/6/timekeeping-linux-vms An introduction to timekeeping in Linux VMs]
[[Category:Time]]
[[Category:Virtual]]
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