===========================
  ARM64 CPU Feature Registers
  ===========================

  
  Author: Suzuki K Poulose <suzuki.poulose@arm.com>
  
  
  This file describes the ABI for exporting the AArch64 CPU ID/feature
  registers to userspace. The availability of this ABI is advertised
  via the HWCAP_CPUID in HWCAPs.
  
  1. Motivation

  -------------

  
  The ARM architecture defines a set of feature registers, which describe
  the capabilities of the CPU/system. Access to these system registers is
  restricted from EL0 and there is no reliable way for an application to
  extract this information to make better decisions at runtime. There is
  limited information available to the application via HWCAPs, however
  there are some issues with their usage.
  
   a) Any change to the HWCAPs requires an update to userspace (e.g libc)
      to detect the new changes, which can take a long time to appear in
      distributions. Exposing the registers allows applications to get the
      information without requiring updates to the toolchains.
  
   b) Access to HWCAPs is sometimes limited (e.g prior to libc, or
      when ld is initialised at startup time).
  
   c) HWCAPs cannot represent non-boolean information effectively. The
      architecture defines a canonical format for representing features
      in the ID registers; this is well defined and is capable of
      representing all valid architecture variations.
  
  
  2. Requirements

  ---------------
  
   a) Safety:

      Applications should be able to use the information provided by the
      infrastructure to run safely across the system. This has greater
      implications on a system with heterogeneous CPUs.
      The infrastructure exports a value that is safe across all the
      available CPU on the system.
  
      e.g, If at least one CPU doesn't implement CRC32 instructions, while
      others do, we should report that the CRC32 is not implemented.
      Otherwise an application could crash when scheduled on the CPU
      which doesn't support CRC32.

   b) Security:

      Applications should only be able to receive information that is
      relevant to the normal operation in userspace. Hence, some of the
      fields are masked out(i.e, made invisible) and their values are set to
      indicate the feature is 'not supported'. See Section 4 for the list
      of visible features. Also, the kernel may manipulate the fields
      based on what it supports. e.g, If FP is not supported by the
      kernel, the values could indicate that the FP is not available
      (even when the CPU provides it).
  
   c) Implementation Defined Features

      The infrastructure doesn't expose any register which is
      IMPLEMENTATION DEFINED as per ARMv8-A Architecture.

   d) CPU Identification:

      MIDR_EL1 is exposed to help identify the processor. On a
      heterogeneous system, this could be racy (just like getcpu()). The
      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::

  
  	/sys/devices/system/cpu/cpu$ID/regs/identification/
  	                                              \- midr
  	                                              \- revidr
  
  3. Implementation
  --------------------
  
  The infrastructure is built on the emulation of the 'MRS' instruction.
  Accessing a restricted system register from an application generates an
  exception and ends up in SIGILL being delivered to the process.
  The infrastructure hooks into the exception handler and emulates the
  operation if the source belongs to the supported system register space.

  The infrastructure emulates only the following system register space::

  	Op0=3, Op1=0, CRn=0, CRm=0,4,5,6,7
  
  (See Table C5-6 'System instruction encodings for non-Debug System
  register accesses' in ARMv8 ARM DDI 0487A.h, for the list of
  registers).
  
  The following rules are applied to the value returned by the
  infrastructure:
  
   a) The value of an 'IMPLEMENTATION DEFINED' field is set to 0.
   b) The value of a reserved field is populated with the reserved
      value as defined by the architecture.
   c) The value of a 'visible' field holds the system wide safe value
      for the particular feature (except for MIDR_EL1, see section 4).
   d) All other fields (i.e, invisible fields) are set to indicate
      the feature is missing (as defined by the architecture).
  
  4. List of registers with visible features
  -------------------------------------------
  
    1) ID_AA64ISAR0_EL1 - Instruction Set Attribute Register 0

  
       +------------------------------+---------+---------+

       | Name                         |  bits   | visible |

       +------------------------------+---------+---------+

       | RNDR                         | [63-60] |    y    |
       +------------------------------+---------+---------+

       | TS                           | [55-52] |    y    |

       +------------------------------+---------+---------+

       | FHM                          | [51-48] |    y    |

       +------------------------------+---------+---------+

       | DP                           | [47-44] |    y    |

       +------------------------------+---------+---------+

       | SM4                          | [43-40] |    y    |

       +------------------------------+---------+---------+

       | SM3                          | [39-36] |    y    |

       +------------------------------+---------+---------+

       | SHA3                         | [35-32] |    y    |

       +------------------------------+---------+---------+

       | RDM                          | [31-28] |    y    |

       +------------------------------+---------+---------+

       | ATOMICS                      | [23-20] |    y    |

       +------------------------------+---------+---------+

       | CRC32                        | [19-16] |    y    |

       +------------------------------+---------+---------+

       | SHA2                         | [15-12] |    y    |

       +------------------------------+---------+---------+

       | SHA1                         | [11-8]  |    y    |

       +------------------------------+---------+---------+

       | AES                          | [7-4]   |    y    |

       +------------------------------+---------+---------+

  
  
    2) ID_AA64PFR0_EL1 - Processor Feature Register 0

  
       +------------------------------+---------+---------+

       | Name                         |  bits   | visible |

       +------------------------------+---------+---------+

       | DIT                          | [51-48] |    y    |

       +------------------------------+---------+---------+

       | SVE                          | [35-32] |    y    |

       +------------------------------+---------+---------+

       | GIC                          | [27-24] |    n    |

       +------------------------------+---------+---------+

       | AdvSIMD                      | [23-20] |    y    |

       +------------------------------+---------+---------+

       | FP                           | [19-16] |    y    |

       +------------------------------+---------+---------+

       | EL3                          | [15-12] |    n    |

       +------------------------------+---------+---------+

       | EL2                          | [11-8]  |    n    |

       +------------------------------+---------+---------+

       | EL1                          | [7-4]   |    n    |

       +------------------------------+---------+---------+

       | EL0                          | [3-0]   |    n    |

       +------------------------------+---------+---------+

    3) ID_AA64PFR1_EL1 - Processor Feature Register 1

       +------------------------------+---------+---------+
       | Name                         |  bits   | visible |
       +------------------------------+---------+---------+

       | MTE                          | [11-8]  |    y    |
       +------------------------------+---------+---------+

       | SSBS                         | [7-4]   |    y    |
       +------------------------------+---------+---------+

       | BT                           | [3-0]   |    y    |
       +------------------------------+---------+---------+

    4) MIDR_EL1 - Main ID Register

       +------------------------------+---------+---------+

       | Name                         |  bits   | visible |

       +------------------------------+---------+---------+

       | Implementer                  | [31-24] |    y    |

       +------------------------------+---------+---------+

       | Variant                      | [23-20] |    y    |

       +------------------------------+---------+---------+

       | Architecture                 | [19-16] |    y    |

       +------------------------------+---------+---------+

       | PartNum                      | [15-4]  |    y    |

       +------------------------------+---------+---------+

       | Revision                     | [3-0]   |    y    |

       +------------------------------+---------+---------+

  
     NOTE: The 'visible' fields of MIDR_EL1 will contain the value
     as available on the CPU where it is fetched and is not a system
     wide safe value.

    5) ID_AA64ISAR1_EL1 - Instruction set attribute register 1

       +------------------------------+---------+---------+

       | Name                         |  bits   | visible |

       +------------------------------+---------+---------+

       | I8MM                         | [55-52] |    y    |
       +------------------------------+---------+---------+
       | DGH                          | [51-48] |    y    |
       +------------------------------+---------+---------+
       | BF16                         | [47-44] |    y    |
       +------------------------------+---------+---------+

       | SB                           | [39-36] |    y    |

       +------------------------------+---------+---------+

       | FRINTTS                      | [35-32] |    y    |

       +------------------------------+---------+---------+

       | GPI                          | [31-28] |    y    |

       +------------------------------+---------+---------+

       | GPA                          | [27-24] |    y    |

       +------------------------------+---------+---------+

       | LRCPC                        | [23-20] |    y    |

       +------------------------------+---------+---------+

       | FCMA                         | [19-16] |    y    |

       +------------------------------+---------+---------+

       | JSCVT                        | [15-12] |    y    |

       +------------------------------+---------+---------+

       | API                          | [11-8]  |    y    |

       +------------------------------+---------+---------+

       | APA                          | [7-4]   |    y    |

       +------------------------------+---------+---------+

       | DPB                          | [3-0]   |    y    |

       +------------------------------+---------+---------+

    6) ID_AA64MMFR2_EL1 - Memory model feature register 2

       +------------------------------+---------+---------+

       | Name                         |  bits   | visible |

       +------------------------------+---------+---------+

       | AT                           | [35-32] |    y    |

       +------------------------------+---------+---------+

    7) ID_AA64ZFR0_EL1 - SVE feature ID register 0

       +------------------------------+---------+---------+

       | Name                         |  bits   | visible |

       +------------------------------+---------+---------+

       | F64MM                        | [59-56] |    y    |
       +------------------------------+---------+---------+
       | F32MM                        | [55-52] |    y    |
       +------------------------------+---------+---------+
       | I8MM                         | [47-44] |    y    |
       +------------------------------+---------+---------+

       | SM4                          | [43-40] |    y    |

       +------------------------------+---------+---------+

       | SHA3                         | [35-32] |    y    |

       +------------------------------+---------+---------+

       | BF16                         | [23-20] |    y    |
       +------------------------------+---------+---------+

       | BitPerm                      | [19-16] |    y    |

       +------------------------------+---------+---------+

       | AES                          | [7-4]   |    y    |

       +------------------------------+---------+---------+

       | SVEVer                       | [3-0]   |    y    |

       +------------------------------+---------+---------+

  Appendix I: Example

  -------------------
  
  ::
  
    /*
     * Sample program to demonstrate the MRS emulation ABI.
     *
     * Copyright (C) 2015-2016, ARM Ltd
     *
     * Author: Suzuki K Poulose <suzuki.poulose@arm.com>
     *
     * This program is free software; you can redistribute it and/or modify
     * it under the terms of the GNU General Public License version 2 as
     * published by the Free Software Foundation.
     *
     * This program is distributed in the hope that it will be useful,
     * but WITHOUT ANY WARRANTY; without even the implied warranty of
     * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     * GNU General Public License for more details.
     * This program is free software; you can redistribute it and/or modify
     * it under the terms of the GNU General Public License version 2 as
     * published by the Free Software Foundation.
     *
     * This program is distributed in the hope that it will be useful,
     * but WITHOUT ANY WARRANTY; without even the implied warranty of
     * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     * GNU General Public License for more details.
     */
  
    #include <asm/hwcap.h>
    #include <stdio.h>
    #include <sys/auxv.h>
  
    #define get_cpu_ftr(id) ({					\

  		unsigned long __val;				\
  		asm("mrs %0, "#id : "=r" (__val));		\
  		printf("%-20s: 0x%016lx
  ", #id, __val);	\
  	})

    int main(void)
    {

  
  	if (!(getauxval(AT_HWCAP) & HWCAP_CPUID)) {
  		fputs("CPUID registers unavailable
  ", stderr);
  		return 1;
  	}
  
  	get_cpu_ftr(ID_AA64ISAR0_EL1);
  	get_cpu_ftr(ID_AA64ISAR1_EL1);
  	get_cpu_ftr(ID_AA64MMFR0_EL1);
  	get_cpu_ftr(ID_AA64MMFR1_EL1);
  	get_cpu_ftr(ID_AA64PFR0_EL1);
  	get_cpu_ftr(ID_AA64PFR1_EL1);
  	get_cpu_ftr(ID_AA64DFR0_EL1);
  	get_cpu_ftr(ID_AA64DFR1_EL1);
  
  	get_cpu_ftr(MIDR_EL1);
  	get_cpu_ftr(MPIDR_EL1);
  	get_cpu_ftr(REVIDR_EL1);

    #if 0

  	/* Unexposed register access causes SIGILL */
  	get_cpu_ftr(ID_MMFR0_EL1);

    #endif

  
  	return 0;

    }