KVM: SVM: Modify AVIC GATag to support max number of 512 vCPUs

Define AVIC_VCPU_ID_MASK based on AVIC_PHYSICAL_MAX_INDEX, i.e. the mask
that effectively controls the largest guest physical APIC ID supported by
x2AVIC, instead of hardcoding the number of bits to 8 (and the number of
VM bits to 24).

The AVIC GATag is programmed into the AMD IOMMU IRTE to provide a
reference back to KVM in case the IOMMU cannot inject an interrupt into a
non-running vCPU.  In such a case, the IOMMU notifies software by creating
a GALog entry with the corresponded GATag, and KVM then uses the GATag to
find the correct VM+vCPU to kick.  Dropping bit 8 from the GATag results
in kicking the wrong vCPU when targeting vCPUs with x2APIC ID > 255.

Fixes: 4d1d7942 ("KVM: SVM: Introduce logic to (de)activate x2AVIC mode")
Cc: stable@vger.kernel.org
Reported-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Tested-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Message-Id: <20230207002156.521736-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/avic.c

@@ -27,19 +27,29 @@
 #include "irq.h"
 #include "svm.h"
 
-/* AVIC GATAG is encoded using VM and VCPU IDs */
-#define AVIC_VCPU_ID_BITS		8
-#define AVIC_VCPU_ID_MASK		((1 << AVIC_VCPU_ID_BITS) - 1)
+/*
+ * Encode the arbitrary VM ID and the vCPU's default APIC ID, i.e the vCPU ID,
+ * into the GATag so that KVM can retrieve the correct vCPU from a GALog entry
+ * if an interrupt can't be delivered, e.g. because the vCPU isn't running.
+ *
+ * For the vCPU ID, use however many bits are currently allowed for the max
+ * guest physical APIC ID (limited by the size of the physical ID table), and
+ * use whatever bits remain to assign arbitrary AVIC IDs to VMs.  Note, the
+ * size of the GATag is defined by hardware (32 bits), but is an opaque value
+ * as far as hardware is concerned.
+ */
+#define AVIC_VCPU_ID_MASK		AVIC_PHYSICAL_MAX_INDEX_MASK
 
-#define AVIC_VM_ID_BITS			24
-#define AVIC_VM_ID_NR			(1 << AVIC_VM_ID_BITS)
-#define AVIC_VM_ID_MASK			((1 << AVIC_VM_ID_BITS) - 1)
+#define AVIC_VM_ID_SHIFT		HWEIGHT32(AVIC_PHYSICAL_MAX_INDEX_MASK)
+#define AVIC_VM_ID_MASK			(GENMASK(31, AVIC_VM_ID_SHIFT) >> AVIC_VM_ID_SHIFT)
 
-#define AVIC_GATAG(x, y)		(((x & AVIC_VM_ID_MASK) << AVIC_VCPU_ID_BITS) | \
+#define AVIC_GATAG(x, y)		(((x & AVIC_VM_ID_MASK) << AVIC_VM_ID_SHIFT) | \
 						(y & AVIC_VCPU_ID_MASK))
-#define AVIC_GATAG_TO_VMID(x)		((x >> AVIC_VCPU_ID_BITS) & AVIC_VM_ID_MASK)
+#define AVIC_GATAG_TO_VMID(x)		((x >> AVIC_VM_ID_SHIFT) & AVIC_VM_ID_MASK)
 #define AVIC_GATAG_TO_VCPUID(x)		(x & AVIC_VCPU_ID_MASK)
 
+static_assert(AVIC_GATAG(AVIC_VM_ID_MASK, AVIC_VCPU_ID_MASK) == -1u);
+
 static bool force_avic;
 module_param_unsafe(force_avic, bool, 0444);