Re: [PATCH][Resend] Fix Warnining in arch/x86/kvm/vmx.c

[Avi Kivity <avi@redhat.com>]

Subrata Modak wrote:
> Hi Avi/Yaniv,
>
> With gcc --version 4.4.1 20090429 (prerelease)
>
> I get the following warning:
> arch/x86/kvm/vmx.c: In function ‘vmx_intr_assist’:
> arch/x86/kvm/vmx.c:3233: warning: ‘max_irr’ may be used uninitialized in this function
> arch/x86/kvm/vmx.c:3233: note: ‘max_irr’ was declared here
>
> Investigation found that:
>
> 3231 static void update_tpr_threshold(struct kvm_vcpu *vcpu)
> 3232 {
> 3233         int max_irr, tpr;
> 3234 
> 3235         if (!vm_need_tpr_shadow(vcpu->kvm))
> 3236                 return;
> 3237 
> 3238         if (!kvm_lapic_enabled(vcpu) ||
> 3239             ((max_irr = kvm_lapic_find_highest_irr(vcpu)) == -1)) {
>
>   

This function no longer exists; can you check if the current code is 
susceptible?

> 	(max_irr = kvm_lapic_find_highest_irr(vcpu)) == -1
>
> may not get a chance to evaluate if:
>
> 	!kvm_lapic_enabled(vcpu)
>
> evaluates to true (as the expressions are Or-ed).
>
> 3240                 vmcs_write32(TPR_THRESHOLD, 0);
> 3241                 return;
> 3242         }
> 3243 
> 3244         tpr = (kvm_lapic_get_cr8(vcpu) & 0x0f) << 4;
> 3245         vmcs_write32(TPR_THRESHOLD, (max_irr > tpr) ? tpr >> 4 : max_irr >> 4);
>
> Using (max_irr > tpr) and max_irr >> 4, without max_irr getting initialized can
> cause trouble.
>   

With !kvm_lapic_enabled(), TPR_THRESHOLD is meaningless.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.