Re: Nested VMX - L1 hangs on running L2

[Jan Kiszka <jan.kiszka@web.de>]

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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


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