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

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

On 06/09/2013 08:27 PM, Gleb Natapov wrote:
> On Sun, Jun 09, 2013 at 08:17:19PM +0800, Xiao Guangrong wrote:
>> On 06/09/2013 07:56 PM, Gleb Natapov wrote:
>>> On Sun, Jun 09, 2013 at 07:44:03PM +0800, Xiao Guangrong wrote:
>>>> 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?
>>>>
>>> I think it means all caches.
>>> 11.6 says:
>>>
>>>   A write to a memory location in a code segment that is currently
>>>   cached in the processor causes the associated cache line (or lines)
>>>   to be invalidated. This check is based on the physical address of
>>>   the instruction. In addition, the P6 family and Pentium processors
>>>   check whether a write to a code segment may modify an instruction that
>>>   has been prefetched for execution. If the write affects a prefetched
>>>   instruction, the prefetch queue is invalidated. This latter check is
>>>   based on the linear address of the instruction. For the Pentium 4 and
>>>   Intel Xeon processors, a write or a snoop of an instruction in a code
>>>   segment, where the target instruction is already decoded and resident in
>>>   the trace cache, invalidates the entire trace cache. The latter behavior
>>>   means that programs that self-modify code can cause severe degradation
>>>   of performance when run on the Pentium 4 and Intel Xeon processors.
>>>
>>> So icache line is invalidate based on physical address so we are OK.
>>
>> Yes.
>>
>>> Prefetched instruction is invalidated based on linear address, but if
>>> all vcpus are in a host guest instruction cannot be prefetched.
>>
>> But what happen if the instruction has been prefetched before vcpu exits
>> to host? Then, after returns to guest, it executes the old instruction.
>>
>> Can it happen?
> I do not thing so, prefetched instructions is not a cache, but I'll ask
> Intel.

Okay, thanks very much for your patience, Gleb!