linux-kernel - Re: [RFC PATCH v1 3/5] KVM: Add paravirt kvm_flush_tlb

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Date:	Mon, 07 May 2012 08:40:10 +0530
From:	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Cc:	peterz@...radead.org, mingo@...e.hu, jeremy@...p.org,
	mtosatti@...hat.com, kvm@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, avi@...hat.com, hpa@...or.com
Subject: Re: [RFC PATCH v1 3/5] KVM: Add paravirt kvm_flush_tlb_others

On Fri, 4 May 2012 17:14:49 +0530, Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com> wrote:
> * Nikunj A. Dadhania <nikunj@...ux.vnet.ibm.com> [2012-04-27 21:54:37]:
> 
> > @@ -1549,6 +1549,11 @@ static void kvm_set_vcpu_state(struct kvm_vcpu *vcpu)
> >  		return;
> > 
> >  	vs->state = 1;
> > +	if (vs->flush_on_enter) {
> > +		kvm_mmu_flush_tlb(vcpu);
> > +		vs->flush_on_enter = 0;
> > +	}
> > +
> >  	kvm_write_guest_cached(vcpu->kvm, ghc, vs, 2*sizeof(__u32));
> 
> Reading flush_on_enter before writing ->state (=1) is racy afaics (and
> may cause vcpu to miss a TLB flush request).
> 
Yes I see this with sysbench, here is what I have now, currently I have
tested it with sysbench(50 runs). Will fold this in my v2.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 60546e9..b2ee9fd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1548,9 +1548,20 @@ static void kvm_set_vcpu_state(struct kvm_vcpu *vcpu)
        if (!(vcpu->arch.v_state.msr_val & KVM_MSR_ENABLED))
                return;
 
+       /* 
+        * Let the guest know that we are online, make sure we do not
+        * overwrite flush_on_enter, just write the vs->state.
+        */
        vs->state = 1;
-       kvm_write_guest_cached(vcpu->kvm, ghc, vs, 2*sizeof(__u32));
+       kvm_write_guest_cached(vcpu->kvm, ghc, vs, 1*sizeof(__u32));
        smp_wmb();
+       /* 
+        * Guest might have seen us offline and would have set
+        * flush_on_enter. 
+        */
+       kvm_read_guest_cached(vcpu->kvm, ghc, vs, 2*sizeof(__u32));
+       if (vs->flush_on_enter) 
+               kvm_x86_ops->tlb_flush(vcpu);
 }
 
 static void kvm_clear_vcpu_state(struct kvm_vcpu *vcpu)


Nikunj

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