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

[Xiao Guangrong <xiaoguangrong.eric@gmail.com>]

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.