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

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Message-ID: <20130609113624.GQ4725@redhat.com>
Date:	Sun, 9 Jun 2013 14:36:24 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Xiao Guangrong <xiaoguangrong.eric@...il.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 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"?

> 
> "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.

--
			Gleb.
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