From bd8fab39cd4fe429f161dfc86bf64993d5c17320 Mon Sep 17 00:00:00 2001 From: Denis Plotnikov Date: Fri, 7 Apr 2017 12:09:53 +0300 Subject: [PATCH] KVM: x86: fix maintaining of kvm_clock stability on guest CPU hotplug MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Reviewed-by: Roman Kagan Signed-off-by: Radim Krčmář --- arch/x86/kvm/x86.c | 29 +++++++++++++++++++---------- 1 file changed, 19 insertions(+), 10 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 422f803fa365..2e4a47275061 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -1454,16 +1454,25 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr) elapsed = ns - kvm->arch.last_tsc_nsec; if (vcpu->arch.virtual_tsc_khz) { - u64 tsc_exp = kvm->arch.last_tsc_write + - 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; + if (data == 0 && msr->host_initiated) { + /* + * detection of vcpu initialization -- need to sync + * with other vCPUs. This particularly helps to keep + * kvm_clock stable after CPU hotplug + */ + synchronizing = true; + } else { + u64 tsc_exp = kvm->arch.last_tsc_write + + 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; + } } /* -- 2.20.1