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

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Date:	Sun, 9 Jun 2013 12:39:10 +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 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.

--
			Gleb.
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