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

[Gleb Natapov]

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.
 
--
			Gleb.
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