Re: [PATCH] x86: kvm: fix unstable_tsc boot

Marcelo Tosatti
2014-02-24T21:56:40Z

On Mon, Feb 24, 2014 at 04:42:29PM +0100, Andrew Jones wrote:
> When the tsc is marked unstable on the host it causes global clock
> updates to be requested each time a vcpu is loaded, nearly halting
> all progress on guests with a large number of vcpus.
>
> Fix this by only requesting a local clock update unless the vcpu
> is migrating to another cpu.
>
> Signed-off-by: Andrew Jones <drjones@redhat.com>
> ---
> arch/x86/kvm/x86.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 6530019116b0d..ea716a162b4a3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2781,15 +2781,18 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> vcpu->arch.last_guest_tsc);
> kvm_x86_ops->write_tsc_offset(vcpu, offset);
> vcpu->arch.tsc_catchup = 1;
> + set_bit(KVM_REQ_CLOCK_UPDATE, &vcpu->requests);
> }
> + }
> +
> + if (unlikely(vcpu->cpu != cpu)) {
> /*
> * On a host with synchronized TSC, there is no need to update
> * kvmclock on vcpu->cpu migration
> */
> if (!vcpu->kvm->arch.use_master_clock || vcpu->cpu == -1)
> kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
> - if (vcpu->cpu != cpu)
> - kvm_migrate_timers(vcpu);
> + kvm_migrate_timers(vcpu);
> vcpu->cpu = cpu;
> }
>
> --
> 1.8.1.4

Consider VCPU1 not doing kvm_arch_vcpu_load (guest not executing HLT,
not switching VCPUs, no exits to QEMU).

VCPU0 doing kvm_arch_vcpu_load (guest executing HLT, say).

The updates on VCPU0 must generate updates on VCPU1 as well, otherwise
NTP correction applies to VCPU0 but not VCPU1.