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KVM: x86: fix maintaining of kvm_clock stability on guest CPU hotplug 

VCPU TSC synchronization is perfromed in kvm_write_tsc() when the TSC
value being set is within 1 second from the expected, as obtained by
extrapolating of the TSC in already synchronized VCPUs.

This is naturally achieved on all VCPUs at VM start and resume;
however on VCPU hotplug it is not: the newly added VCPU is created
with TSC == 0 while others are well ahead.

To compensate for that, consider host-initiated kvm_write_tsc() with
TSC == 0 a special case requiring synchronization regardless of the
current TSC on other VCPUs.

Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
This commit is contained in:
Denis Plotnikov 2017-04-07 12:09:53 +03:00 committed by Radim Krčmář
parent c5e8ec8e9b
commit bd8fab39cd
1 changed files with 19 additions and 10 deletions
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29
arch/x86/kvm/x86.c
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@ -1454,16 +1454,25 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
elapsed = ns - kvm->arch.last_tsc_nsec; elapsed = ns - kvm->arch.last_tsc_nsec;
if (vcpu->arch.virtual_tsc_khz) { if (vcpu->arch.virtual_tsc_khz) {
u64 tsc_exp = kvm->arch.last_tsc_write + if (data == 0 && msr->host_initiated) {
nsec_to_cycles(vcpu, elapsed); /*
u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL; * detection of vcpu initialization -- need to sync
/* * with other vCPUs. This particularly helps to keep
* Special case: TSC write with a small delta (1 second) * kvm_clock stable after CPU hotplug
* of virtual cycle time against real time is */
* interpreted as an attempt to synchronize the CPU. synchronizing = true;
*/ } else {
synchronizing = data < tsc_exp + tsc_hz && u64 tsc_exp = kvm->arch.last_tsc_write +
data + tsc_hz > tsc_exp; nsec_to_cycles(vcpu, elapsed);
u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL;
/*
* Special case: TSC write with a small delta (1 second)
* of virtual cycle time against real time is
* interpreted as an attempt to synchronize the CPU.
*/
synchronizing = data < tsc_exp + tsc_hz &&
data + tsc_hz > tsc_exp;
}
} }
/* /*