Re: Nested VMX - L1 hangs on running L2

Marcelo Tosatti
2011-07-20T16:36:06Z

On Wed, Jul 20, 2011 at 06:19:19PM +0200, Jan Kiszka wrote:
> On 2011-07-20 18:12, Marcelo Tosatti wrote:
> > On Wed, Jul 20, 2011 at 09:58:47AM +0200, Jan Kiszka wrote:
> >> On 2011-07-18 20:26, Marcelo Tosatti 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 :
> >>>>
> >>>> ...
> >>>> qemu-kvm-19756 [013] 153774.856178: kvm_exit: reason APIC_ACCESS rip 0xffffffff81025098 info 1380 0
> >>>> qemu-kvm-19756 [013] 153774.856189: kvm_exit: reason VMREAD rip 0xffffffffa00d5127 info 0 0
> >>>> qemu-kvm-19756 [013] 153774.856191: kvm_exit: reason VMREAD rip 0xffffffffa00d5127 info 0 0
> >>>> ...
> >>>>
> >>>> My point being that I only see kvm_exit messages but no kvm_entry. Does this mean that the VCPUs
> >>>> are somehow stuck in L2 ?
> >>>>
> >>>> Anyway, since this setup was running fine for me on older kernels, and I couldn't
> >>>> identify any significant changes in nVMX, I sifted through the other KVM changes and found this :
> >>>>
> >>>> --
> >>>> commit 1aa8ceef0312a6aae7dd863a120a55f1637b361d
> >>>> Author: Nikola Ciprich <extmaillist@linuxbox.cz>
> >>>> Date: Wed Mar 9 23:36:51 2011 +0100
> >>>>
> >>>> KVM: fix kvmclock regression due to missing clock update
> >>>>
> >>>> commit 387b9f97750444728962b236987fbe8ee8cc4f8c moved kvm_request_guest_time_update(vcpu),
> >>>> breaking 32bit SMP guests using kvm-clock. Fix this by moving (new) clock update function
> >>>> to proper place.
> >>>>
> >>>> Signed-off-by: Nikola Ciprich <nikola.ciprich@linuxbox.cz>
> >>>> Acked-by: Zachary Amsden <zamsden@redhat.com>
> >>>> Signed-off-by: Avi Kivity <avi@redhat.com>
> >>>>
> >>>> index 01f08a6..f1e4025 100644 (file)
> >>>> --- a/arch/x86/kvm/x86.c
> >>>> +++ b/arch/x86/kvm/x86.c
> >>>> @@ -2127,8 +2127,8 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> >>>> if (check_tsc_unstable()) {
> >>>> kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
> >>>> vcpu->arch.tsc_catchup = 1;
> >>>> - kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> >>>> }
> >>>> + kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> >>>> if (vcpu->cpu != cpu)
> >>>> kvm_migrate_timers(vcpu);
> >>>> vcpu->cpu = cpu;
> >>>> --
> >>>>
> >>>> If I revert this change, my L1/L2 guests run fine. This ofcourse, just hides the bug
> >>>> because on my machine, check_tsc_unstable() returns false.
> >>>>
> >>>> I found out from Nadav that when KVM decides to run L2, it will write
> >>>> vmcs01->tsc_offset + vmcs12->tsc_offset to the active TSC_OFFSET which seems right.
> >>>> But I verified that, if instead, I just write
> >>>> vmcs01->tsc_offset to TSC_OFFSET in prepare_vmcs02(), I don't see the bug anymore.
> >>>>
> >>>> Not sure where to go from here. I would appreciate if any one has any ideas.
> >>>>
> >>>>
> >>>> Bandan
> >>>
> >>> 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();
> >>
> >> That unfortunately does not fix the L1 lockups I get here - unless I
> >> confine L1 to a single CPU. It looks like (don't have all symbols for
> >> the guest kernel ATM) that we are stuck in processing a timer IRQ.
> >>
> >> Jan
> >
> > Is L1 using kvmclock?
>
> Yes, it's a standard 3.0-rc7 SUSE kernel. Disabling it seems to help on
> first glance.
>
> Jan

The patch i posted is broken, thinking on a proper fix.