Re: [PATCH] x86: handle double and triple faults for every exception

[Jan Kiszka <jan.kiszka@siemens.com>]

Avi Kivity wrote:
> Jan Kiszka wrote:
>> Joerg Roedel wrote:
>>  
>>> The current KVM x86 exception code handles double and triple faults
>>> only for
>>> page fault exceptions. This patch extends this detection for every
>>> exception
>>> that gets queued for the guest.
>>>
>>> Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
>>> Cc: Jan Kiszka <jan.kiszka@siemens.com>
>>> ---
>>>  arch/x86/kvm/x86.c |   31 +++++++++++++++++--------------
>>>  1 files changed, 17 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 578a0c1..c05aa32 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -144,9 +144,21 @@ void kvm_set_apic_base(struct kvm_vcpu *vcpu,
>>> u64 data)
>>>  }
>>>  EXPORT_SYMBOL_GPL(kvm_set_apic_base);
>>>  
>>> +static void handle_multiple_faults(struct kvm_vcpu *vcpu)
>>> +{
>>> +    if (vcpu->arch.exception.nr != DF_VECTOR) {
>>> +        vcpu->arch.exception.nr = DF_VECTOR;
>>> +        vcpu->arch.exception.error_code = 0;
>>> +    } else
>>> +        set_bit(KVM_REQ_TRIPLE_FAULT, &vcpu->requests);
>>> +}
>>> +
>>>  void kvm_queue_exception(struct kvm_vcpu *vcpu, unsigned nr)
>>>  {
>>> -    WARN_ON(vcpu->arch.exception.pending);
>>> +    if (vcpu->arch.exception.pending) {
>>> +        handle_multiple_faults(vcpu);
>>> +        return;
>>> +    }
>>>      vcpu->arch.exception.pending = true;
>>>      vcpu->arch.exception.has_error_code = false;
>>>      vcpu->arch.exception.nr = nr;
>>> @@ -157,25 +169,16 @@ void kvm_inject_page_fault(struct kvm_vcpu
>>> *vcpu, unsigned long addr,
>>>                 u32 error_code)
>>>  {
>>>      ++vcpu->stat.pf_guest;
>>> -    if (vcpu->arch.exception.pending) {
>>> -        if (vcpu->arch.exception.nr == PF_VECTOR) {
>>> -            printk(KERN_DEBUG "kvm: inject_page_fault:"
>>> -                    " double fault 0x%lx\n", addr);
>>> -            vcpu->arch.exception.nr = DF_VECTOR;
>>> -            vcpu->arch.exception.error_code = 0;
>>> -        } else if (vcpu->arch.exception.nr == DF_VECTOR) {
>>> -            /* triple fault -> shutdown */
>>> -            set_bit(KVM_REQ_TRIPLE_FAULT, &vcpu->requests);
>>> -        }
>>> -        return;
>>> -    }
>>>      vcpu->arch.cr2 = addr;
>>>      kvm_queue_exception_e(vcpu, PF_VECTOR, error_code);
>>>  }
>>>  
>>>  void kvm_queue_exception_e(struct kvm_vcpu *vcpu, unsigned nr, u32
>>> error_code)
>>>  {
>>> -    WARN_ON(vcpu->arch.exception.pending);
>>> +    if (vcpu->arch.exception.pending) {
>>> +        handle_multiple_faults(vcpu);
>>> +        return;
>>> +    }
>>>      vcpu->arch.exception.pending = true;
>>>      vcpu->arch.exception.has_error_code = true;
>>>      vcpu->arch.exception.nr = nr;
>>>     
>>
>> And here is an add-on patch to fix reset-on-triple-fault:
>>
>>
>> Clear the pending original exception when raising a triple fault. This
>> allows to re-use the vcpu instance, e.g. after a reset which is
>> typically issued as reaction on the triple fault.
>>
>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>
>> ---
>>  arch/x86/kvm/x86.c |    4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> Index: b/arch/x86/kvm/x86.c
>> ===================================================================
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -149,8 +149,10 @@ static void handle_multiple_faults(struc
>>      if (vcpu->arch.exception.nr != DF_VECTOR) {
>>          vcpu->arch.exception.nr = DF_VECTOR;
>>          vcpu->arch.exception.error_code = 0;
>> -    } else
>> +    } else {
>>          set_bit(KVM_REQ_TRIPLE_FAULT, &vcpu->requests);
>> +        vcpu->arch.exception.pending = false;
>> +    }
>>  }
>>  
>>  void kvm_queue_exception(struct kvm_vcpu *vcpu, unsigned nr)
>>
>>   
> 
> There's a bigger problem here.  The exception queue is hidden state that
> qemu and load and save.

Could you elaborate a bit on what the problematic scenario precisely is
(that pending triple faults would not be saved/restored while pending
exceptions are?), and if I/we can do anything to resolve it?

Jan

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux

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