Messages in this thread |  | | | Date | Wed, 16 Jun 2010 12:36:17 -1000 | | From | Zachary Amsden <> | | Subject | Re: [PATCH 16/17] TSC reset compensation |
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On 06/16/2010 03:52 AM, Glauber Costa wrote: > On Mon, Jun 14, 2010 at 09:34:18PM -1000, Zachary Amsden wrote: > >> Attempt to synchronize TSCs which are reset to the same value. In the >> case of a reliable hardware TSC, we can just re-use the same offset, but >> on non-reliable hardware, we can get closer by adjusting the offset to >> match the elapsed time. >> >> Signed-off-by: Zachary Amsden<zamsden@redhat.com> >> --- >> arch/x86/kvm/x86.c | 34 ++++++++++++++++++++++++++++++++-- >> 1 files changed, 32 insertions(+), 2 deletions(-) >> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> index 8e836e9..cedb71f 100644 >> --- a/arch/x86/kvm/x86.c >> +++ b/arch/x86/kvm/x86.c >> @@ -937,14 +937,44 @@ static inline void kvm_request_guest_time_update(struct kvm_vcpu *v) >> set_bit(KVM_REQ_CLOCK_SYNC,&v->requests); >> } >> >> +static inline int kvm_tsc_reliable(void) >> +{ >> + return (boot_cpu_has(X86_FEATURE_CONSTANT_TSC)&& >> + boot_cpu_has(X86_FEATURE_NONSTOP_TSC)&& >> + !check_tsc_unstable()); >> +} >> + >> > why can't we re-use vmware TSC_RELIABLE flag? >
It's only set for VMware. Basically, it means "you are running in a VMware hypervisor, TSC is reliable". Which KVM won't ever be, at least, not in production use, so it doesn't make that sort of sense here. Besides, a system with a reliable TSC can become a system without a reliable TSC : CPU hotplug will always guarantee this.
We could, however, have the guest set the TSC_RELIABLE flag for itself if KVM somehow makes that promise (currently, it does not).
Zach
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