linux-kernel - Re: [PATCH] kvm/x86: check KVM_REQ_TLB

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Date:	Mon, 16 Mar 2009 15:11:34 +0100
From:	Joerg Roedel <joerg.roedel@....com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm/x86: check KVM_REQ_TLB_FLUSH with irqs disabled

On Mon, Mar 16, 2009 at 03:12:52PM +0200, Avi Kivity wrote:
> Joerg Roedel wrote:
> >For KVM remote TLB flushes we need to check the KVM_REQ_TLB_FLUSH
> >request flag when the irqs are already disabled. Otherwise there is a
> >small window of time for a race condition where we may enter a guest
> >without doing a requested TLB flush.
> >
> >@@ -3108,8 +3108,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
> > 			kvm_write_guest_time(vcpu);
> > 		if (test_and_clear_bit(KVM_REQ_MMU_SYNC, &vcpu->requests))
> > 			kvm_mmu_sync_roots(vcpu);
> >-		if (test_and_clear_bit(KVM_REQ_TLB_FLUSH, &vcpu->requests))
> >-			kvm_x86_ops->tlb_flush(vcpu);
> > 		if (test_and_clear_bit(KVM_REQ_REPORT_TPR_ACCESS,
> > 				       &vcpu->requests)) {
> > 			kvm_run->exit_reason = KVM_EXIT_TPR_ACCESS;
> >@@ -3133,6 +3131,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
> >  	local_irq_disable();
> > +	if (test_and_clear_bit(KVM_REQ_TLB_FLUSH, &vcpu->requests))
> >+		kvm_x86_ops->tlb_flush(vcpu);
> >+
> > 	if (vcpu->requests || need_resched() || signal_pending(current)) {
> > 		local_irq_enable();
> > 		preempt_enable();
> >  
> 
> If we lost the race and someone sets a bit after the test, then the
> test immediately above will pick this up retry the bit tests.
>

Ah true. Sorry for the noise.

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