On 04/28/2011 12:06 AM, Jan Kiszka wrote:
On 2011-04-28 08:59, Zachary Amsden wrote:So I've been going over the new code changes to the TSC related code and I don't like one particular set of changes. In particular, here: kvm_x86_ops->vcpu_load(vcpu, cpu); if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) { /* Make sure TSC doesn't go backwards */ s64 tsc_delta; u64 tsc; kvm_get_msr(vcpu, MSR_IA32_TSC,&tsc); tsc_delta = !vcpu->arch.last_guest_tsc ? 0 : tsc - vcpu->arch.last_guest_tsc; if (tsc_delta< 0) mark_tsc_unstable("KVM discovered backwards TSC"); if (check_tsc_unstable()) { kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta); vcpu->arch.tsc_catchup = 1; } The point of this code fragment is to test the host clock to see if it is stable, because we may have just come back from an idle phase which stopped the TSC, switched CPUs, or come back from a deep sleep state which reset the host TSC. However, the above code is fetching the guest TSC instead of the host TSC, which isn't the way it is supposed to work. I saw a patch floating around that touched this code recently, but I think there's a definite issue here that needs addressing.And /me still wonders (like I did when this first popped up) if the proper place of determining TSC stability really have to be KVM.
No, it's not. I have a patch which fixes that. Was in the process of merging it into my local tree when I found this.
If the Linux core fails to detect some instability and KVM has to jump in, shouldn't we better improve the core's detection abilities and make use of them in KVM? Conceptually this looks like we are currently just working around a core deficit in KVM.
100% correct you are. Zach -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
