linux-kernel - Re: [PATCH v3 00/26] KVM: x86: Halt and APICv overhaul

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Message-ID: <YbIklNHIFnREGFAp@google.com>
Date:   Thu, 9 Dec 2021 15:45:24 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        kvm@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 00/26] KVM: x86: Halt and APICv overhaul

On Thu, Dec 09, 2021, Maxim Levitsky wrote:
> On Thu, 2021-12-09 at 15:29 +0100, Paolo Bonzini wrote:
> > On 12/9/21 01:02, Sean Christopherson wrote:
> > > RDX, a.k.a. ir_data is NULL.  This check in svm_ir_list_add()
> > > 
> > > 	if (pi->ir_data && (pi->prev_ga_tag != 0)) {
> > > 
> > > implies pi->ir_data can be NULL, but neither avic_update_iommu_vcpu_affinity()
> > > nor amd_iommu_update_ga() check ir->data for NULL.
> > > 
> > > amd_ir_set_vcpu_affinity() returns "success" without clearing pi.is_guest_mode
> > > 
> > > 	/* Note:
> > > 	 * This device has never been set up for guest mode.
> > > 	 * we should not modify the IRTE
> > > 	 */
> > > 	if (!dev_data || !dev_data->use_vapic)
> > > 		return 0;
> > > 
> > > so it's plausible svm_ir_list_add() could add to the list with a NULL pi->ir_data.
> > > 
> > > But none of the relevant code has seen any meaningful changes since 5.15, so odds
> > > are good I broke something :-/
> 
> Doesn't reproduce here yet even with my iommu changes :-(
> Oh well.

Hmm, which suggests it could be an existing corner case.

Based on the above, this seems prudent and correct:

@@ -747,7 +754,7 @@ static int svm_ir_list_add(struct vcpu_svm *svm, struct amd_iommu_pi_data *pi)
         * so we need to check here if it's already been * added
         * to the ir_list.
         */
-       if (pi->ir_data && (pi->prev_ga_tag != 0)) {
+       if (pi->prev_ga_tag != 0) {
                struct kvm *kvm = svm->vcpu.kvm;
                u32 vcpu_id = AVIC_GATAG_TO_VCPUID(pi->prev_ga_tag);
                struct kvm_vcpu *prev_vcpu = kvm_get_vcpu_by_id(kvm, vcpu_id);
@@ -877,7 +884,7 @@ int svm_update_pi_irte(struct kvm *kvm, unsigned int host_irq,
                         * we can reference to them directly when we update vcpu
                         * scheduling information in IOMMU irte.
                         */
-                       if (!ret && pi.is_guest_mode)
+                       if (!ret && pi.is_guest_mode && pi.ir_data)
                                svm_ir_list_add(svm, &pi);
                } else {
                        /* Use legacy mode in IRTE */
@@ -898,7 +905,7 @@ int svm_update_pi_irte(struct kvm *kvm, unsigned int host_irq,
                         * was cached. If so, we need to clean up the per-vcpu
                         * ir_list.
                         */
-                       if (!ret && pi.prev_ga_tag) {
+                       if (!ret && pi.prev_ga_tag && !WARN_ON(!pi.ir_data)) {
                                int id = AVIC_GATAG_TO_VCPUID(pi.prev_ga_tag);
                                struct kvm_vcpu *vcpu;