From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Subject: Re: [PATCH][Resend] Fix Warnining in arch/x86/kvm/vmx.c
Date: Wed, 13 May 2009 12:41:02 +0300
Message-ID: <4A0A95AE.9090008@redhat.com>
References: <20090513091643.8216.46699.sendpatchset@subratamodak.linux.ibm.com>
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Cc: kvm@vger.kernel.org, Yaniv Kamay <yaniv@redhat.com>,
	Sachin P Sant <sachinp@linux.vnet.ibm.com>,
	Balbir Singh <balbir@linux.vnet.ibm.com>
To: Subrata Modak <subrata@linux.vnet.ibm.com>
Return-path: <kvm-owner@vger.kernel.org>
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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 =E2=80=98vmx_intr_assist=E2=80=99:
> arch/x86/kvm/vmx.c:3233: warning: =E2=80=98max_irr=E2=80=99 may be us=
ed uninitialized in this function
> arch/x86/kvm/vmx.c:3233: note: =E2=80=98max_irr=E2=80=99 was declared=
 here
>
> Investigation found that:
>
> 3231 static void update_tpr_threshold(struct kvm_vcpu *vcpu)
> 3232 {
> 3233         int max_irr, tpr;
> 3234=20
> 3235         if (!vm_need_tpr_shadow(vcpu->kvm))
> 3236                 return;
> 3237=20
> 3238         if (!kvm_lapic_enabled(vcpu) ||
> 3239             ((max_irr =3D kvm_lapic_find_highest_irr(vcpu)) =3D=3D=
 -1)) {
>
>  =20

This function no longer exists; can you check if the current code is=20
susceptible?

> 	(max_irr =3D kvm_lapic_find_highest_irr(vcpu)) =3D=3D -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=20
> 3244         tpr =3D (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 initi=
alized can
> cause trouble.
>  =20

With !kvm_lapic_enabled(), TPR_THRESHOLD is meaningless.


--=20
Do not meddle in the internals of kernels, for they are subtle and quic=
k to panic.