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Date:   Wed, 2 Feb 2022 16:05:32 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     "Suthikulpanit, Suravee" <suravee.suthikulpanit@amd.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org, x86@kernel.org,
        pbonzini@redhat.com, joro@8bytes.org, mlevitsk@redhat.com,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        peterz@infradead.org, hpa@zytor.com, thomas.lendacky@amd.com,
        jon.grimm@amd.com
Subject: Re: [PATCH v3 3/3] KVM: SVM: Extend host physical APIC ID field to
 support more than 8-bit
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On Wed, Feb 02, 2022, Sean Christopherson wrote:
> On Wed, Feb 02, 2022, Suthikulpanit, Suravee wrote:
> > As I mentioned, the APM will be corrected to remove the word "x2APIC".
> 
> Ah, I misunderstood what part was being amended.
> 
> > Essentially, it will be changed to:
> > 
> >  * 7:0  - For systems w/ max APIC ID upto 255 (a.k.a old system)
> >  * 11:8 - For systems w/ max APIC ID 256 and above (a.k.a new system). Otherwise, reserved and should be zero.
> > 
> > As for the required number of bits, there is no good way to tell what's the max
> > APIC ID would be on a particular system. Hence, we utilize the apic_get_max_phys_apicid()
> > to figure out how to properly program the table (which is leaving the reserved field
> > alone when making change to the table).
> > 
> > The avic_host_physical_id_mask is not just for protecting APIC ID larger than
> > the allowed fields. It is also currently used for clearing the old physical APIC ID table entry
> > before programing it with the new APIC ID.
> 
> Just clear 11:0 unconditionally, the reserved bits are Should Be Zero.

Actually, that needs to be crafted a bug fix that's sent to stable@, otherwise
running old kernels on new hardware will break.  I'm pretty sure this is the
entirety of what's needed.

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 90364d02f22a..e4cfd8bf4f24 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -974,17 +974,12 @@ avic_update_iommu_vcpu_affinity(struct kvm_vcpu *vcpu, int cpu, bool r)
 void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 {
        u64 entry;
-       /* ID = 0xff (broadcast), ID > 0xff (reserved) */
        int h_physical_id = kvm_cpu_get_apicid(cpu);
        struct vcpu_svm *svm = to_svm(vcpu);

        lockdep_assert_preemption_disabled();

-       /*
-        * Since the host physical APIC id is 8 bits,
-        * we can support host APIC ID upto 255.
-        */
-       if (WARN_ON(h_physical_id > AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK))
+       if (WARN_ON(h_physical_id & ~AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK))
                return;

        /*
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 73525353e424..a157af1cce6a 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -560,7 +560,7 @@ extern struct kvm_x86_nested_ops svm_nested_ops;
 #define AVIC_LOGICAL_ID_ENTRY_VALID_BIT                        31
 #define AVIC_LOGICAL_ID_ENTRY_VALID_MASK               (1 << 31)

-#define AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK   (0xFFULL)
+#define AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK   GENMASK_ULL(11:0)
 #define AVIC_PHYSICAL_ID_ENTRY_BACKING_PAGE_MASK       (0xFFFFFFFFFFULL << 12)
 #define AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK         (1ULL << 62)
 #define AVIC_PHYSICAL_ID_ENTRY_VALID_MASK              (1ULL << 63)