Re: Nested VMX - L1 hangs on running L2

Bandan Das
2011-07-19T02:41:52Z

On 0, Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
> On Fri, Jul 08, 2011 at 02:40:53PM -0400, Bandan Das wrote:
> > I have already discussed this a bit with Nadav but hoping someone
> > else has any other ideas/clues/suggestions/comments. With recent versions of the
> > kernel (The last I tried is 3.0-rc5 with nVMX patches already merged), my L1 guest
> > always hangs when I start L2.
> >
> > My setup : The host, L1 and L2 all are FC15 with the host running 3.0-rc5. When L1 is up
> > and running, I start L2 from L1. Within a minute or two, both L1 and L2 hang. Although, if
> > if I run tracing on the host, I see :
> >
>
...
> Using guests TSC value when performing TSC adjustments is wrong. Can
> you please try the following patch, which skips TSC adjustments if
> vcpu is in guest mode.
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2b76ae3..44c90d1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1096,6 +1096,9 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
> s64 kernel_ns, max_kernel_ns;
> u64 tsc_timestamp;
>
> + if (is_guest_mode(v))
> + return 0;
> +
> /* Keep irq disabled to prevent changes to the clock */
> local_irq_save(flags);
> kvm_get_msr(v, MSR_IA32_TSC, &tsc_timestamp);
> @@ -2214,6 +2217,9 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> tsc_delta = !vcpu->arch.last_guest_tsc ? 0 :
> tsc - vcpu->arch.last_guest_tsc;
>
> + if (is_guest_mode(vcpu))
> + tsc_delta = 0;
> +
> if (tsc_delta < 0)
> mark_tsc_unstable("KVM discovered backwards TSC");
> if (check_tsc_unstable()) {
> @@ -2234,7 +2240,8 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> {
> kvm_x86_ops->vcpu_put(vcpu);
> kvm_put_guest_fpu(vcpu);
> - kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc);
> + if (!is_guest_mode(vcpu))
> + kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc);
> }
>
> static int is_efer_nx(void)
> @@ -5717,7 +5724,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> if (hw_breakpoint_active())
> hw_breakpoint_restore();
>
> - kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc);
> + if (!is_guest_mode(vcpu))
> + kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc);
>
> vcpu->mode = OUTSIDE_GUEST_MODE;
> smp_wmb();

Hi Marcelo,

Thanks for looking at this and for the explanation of the cause. Your patch does solve
my problem. I have been running my L2 guest for a few hours now without any hang, while
usually, I would get a hang within minutes of booting L2.

Thanks again!
Bandan