arm64: Expose AIDR_EL1 via sysfs

The KVM PV ABI recently added a feature that allows the VM to discover
the set of physical CPU implementations, identified by a tuple of
{MIDR_EL1, REVIDR_EL1, AIDR_EL1}. Unlike other KVM PV features, the
expectation is that the VMM implements the hypercall instead of KVM as
it has the authoritative view of where the VM gets scheduled.

To do this the VMM needs to know the values of these registers on any
CPU in the system. While MIDR_EL1 and REVIDR_EL1 are already exposed,
AIDR_EL1 is not. Provide it in sysfs along with the other identification
registers.

Signed-off-by: Oliver Upton <[e-mail]>
Reviewed-by: Cornelia Huck <[e-mail]>
Reviewed-by: Anshuman Khandual <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Will Deacon <[e-mail]>

Documentation/ABI/testing/sysfs-devices-system-cpu

···
 		/sys/devices/system/cpu/cpuX/regs/identification/
 		/sys/devices/system/cpu/cpuX/regs/identification/midr_el1
 		/sys/devices/system/cpu/cpuX/regs/identification/revidr_el1
+		/sys/devices/system/cpu/cpuX/regs/identification/aidr_el1
 		/sys/devices/system/cpu/cpuX/regs/identification/smidr_el1
 Date:		June 2016
 Contact:	Linux ARM Kernel Mailing list <[e-mail]>

Documentation/arch/arm64/cpu-feature-registers.rst

···
     process could be migrated to another CPU by the time it uses the
     register value, unless the CPU affinity is set. Hence, there is no
     guarantee that the value reflects the processor that it is
-    currently executing on. The REVIDR is not exposed due to this
-    constraint, as REVIDR makes sense only in conjunction with the
-    MIDR. Alternately, MIDR_EL1 and REVIDR_EL1 are exposed via sysfs
-    at::
+    currently executing on. REVIDR and AIDR are not exposed due to this
+    constraint, as these registers only make sense in conjunction with
+    the MIDR. Alternately, MIDR_EL1, REVIDR_EL1, and AIDR_EL1 are exposed
+    via sysfs at::
 
 	/sys/devices/system/cpu/cpu$ID/regs/identification/
-	                                              \- midr
-	                                              \- revidr
+	                                              \- midr_el1
+	                                              \- revidr_el1
+	                                              \- aidr_el1
 
 3. Implementation
 --------------------

arch/arm64/include/asm/cpu.h

···
 	u64		reg_dczid;
 	u64		reg_midr;
 	u64		reg_revidr;
+	u64		reg_aidr;
 	u64		reg_gmid;
 	u64		reg_smidr;
 	u64		reg_mpamidr;

arch/arm64/kernel/cpuinfo.c

···
 
 CPUREGS_ATTR_RO(midr_el1, midr);
 CPUREGS_ATTR_RO(revidr_el1, revidr);
+CPUREGS_ATTR_RO(aidr_el1, aidr);
 CPUREGS_ATTR_RO(smidr_el1, smidr);
 
 static struct attribute *cpuregs_id_attrs[] = {
 	&cpuregs_attr_midr_el1.attr,
 	&cpuregs_attr_revidr_el1.attr,
+	&cpuregs_attr_aidr_el1.attr,
 	NULL
 };
 
···
 	info->reg_dczid = read_cpuid(DCZID_EL0);
 	info->reg_midr = read_cpuid_id();
 	info->reg_revidr = read_cpuid(REVIDR_EL1);
+	info->reg_aidr = read_cpuid(AIDR_EL1);
 
 	info->reg_id_aa64dfr0 = read_cpuid(ID_AA64DFR0_EL1);
 	info->reg_id_aa64dfr1 = read_cpuid(ID_AA64DFR1_EL1);