Re: [PATCH] kvm/x86: check KVM_REQ_TLB_FLUSH with irqs disabled

[Avi Kivity <avi@redhat.com>]

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.


-- 
error compiling committee.c: too many arguments to function