linux-kernel - Re: [PATCH] KVM: x86: fix missed memory synchronization when patch hypercall

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Message-ID: <51B46A83.4010008@gmail.com>
Date:	Sun, 09 Jun 2013 19:44:03 +0800
From:	Xiao Guangrong <xiaoguangrong.eric@...il.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Paolo Bonzini <pbonzini@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: fix missed memory synchronization when patch
 hypercall

On 06/09/2013 07:36 PM, Gleb Natapov wrote:
> On Sun, Jun 09, 2013 at 07:25:17PM +0800, Xiao Guangrong wrote:
>> On 06/09/2013 06:19 PM, Gleb Natapov wrote:
>>> On Sun, Jun 09, 2013 at 06:01:45PM +0800, Xiao Guangrong wrote:
>>>> On 06/09/2013 05:39 PM, Gleb Natapov wrote:
>>>>> On Sun, Jun 09, 2013 at 05:29:37PM +0800, Xiao Guangrong wrote:
>>>>>> On 06/09/2013 04:45 PM, Gleb Natapov wrote:
>>>>>>
>>>>>>> +static int emulator_fix_hypercall(struct x86_emulate_ctxt *ctxt)
>>>>>>> +{
>>>>>>> +	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
>>>>>>> +	return kvm_exec_with_stopped_vcpu(vcpu->kvm,
>>>>>>> +			emulator_fix_hypercall_cb, ctxt);
>>>>>>> +}
>>>>>>> +
>>>>>>> +
>>>>>>>  /*
>>>>>>>   * Check if userspace requested an interrupt window, and that the
>>>>>>>   * interrupt window is open.
>>>>>>> @@ -5761,6 +5769,10 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>>>>>>  			kvm_deliver_pmi(vcpu);
>>>>>>>  		if (kvm_check_request(KVM_REQ_SCAN_IOAPIC, vcpu))
>>>>>>>  			vcpu_scan_ioapic(vcpu);
>>>>>>> +		if (kvm_check_request(KVM_REQ_STOP_VCPU, vcpu)){
>>>>>>> +			mutex_lock(&vcpu->kvm->lock);
>>>>>>> +			mutex_unlock(&vcpu->kvm->lock);
>>>>>>
>>>>>> We should execute a serializing instruction here?
>>>>>>
>>>>>>> --- a/virt/kvm/kvm_main.c
>>>>>>> +++ b/virt/kvm/kvm_main.c
>>>>>>> @@ -222,6 +222,18 @@ void kvm_make_scan_ioapic_request(struct kvm *kvm)
>>>>>>>  	make_all_cpus_request(kvm, KVM_REQ_SCAN_IOAPIC);
>>>>>>>  }
>>>>>>>
>>>>>>> +int kvm_exec_with_stopped_vcpu(struct kvm *kvm, int (*cb)(void *), void *data)
>>>>>>> +{
>>>>>>> +	int r;
>>>>>>> +
>>>>>>> +	mutex_lock(&kvm->lock);
>>>>>>> +	make_all_cpus_request(kvm, KVM_REQ_STOP_VCPU);
>>>>>>> +	r = cb(data);
>>>>>>
>>>>>> And here?
>>>>> Since the serialisation instruction the SDM suggest to use is CPUID I
>>>>> think the point here is to flush CPU pipeline. Since all vcpus are out
>>>>> of a guest mode I think out of order execution of modified instruction
>>>>> is no an issue here.
>>>>
>>>> I checked the SDM that it did not said VMLAUNCH/VMRESUME are the
>>>> serializing instructions both in VM-Entry description and Instruction
>>>> reference, instead it said the VMX related serializing instructions are:
>>>> INVEPT, INVVPID.
>>>>
>>>> So, i guess the explicit serializing instruction is needed here.
>>>>
>>> Again the question is what for? SDM says:
>>>
>>>   The Intel 64 and IA-32 architectures define several serializing
>>>   instructions. These instructions force the processor to complete all
>>>   modifications to flags, registers, and memory by previous instructions
>>>   and to drain all buffered writes to memory before the next instruction
>>>   is fetched and executed.
>>>
>>> So flags and registers modifications on a host are obviously irrelevant for a guest.
>>
>> Okay. Hmm... but what can guarantee that "drain all buffered writes to memory"?
> Memory barrier should guaranty that as I said bellow.
> 
>>
>>> And for memory ordering we have smp_mb() on a guest entry.
>>
>> If i understand the SDM correctly, memory-ordering instructions can not drain
>> instruction buffer, it only drains "data memory subsystem":
> What is "instruction buffer"?

I mean "Instruction Cache" (icache). Can memory ordering drain icache?
The "data memory subsystem" confused me, does it mean dcache?

> 
>>
>> "The following instructions are memory-ordering instructions, not serializing instruc-
>> tions. These drain the data memory subsystem. They do not serialize the instruction
>> execution stream:"
>>
>> No?
> Yes, but we have no issue with instruction execution stream as I said
> above. No guest instruction can be in a pipeline while all vcpus are in
> a host.



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