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)
