arm64/sve: Add documentation

            Scalable Vector Extension support for AArch64 Linux

            ===================================================

Author: Dave Martin <Dave.Martin@arm.com>

Date:   4 August 2017

This document outlines briefly the interface provided to userspace by Linux in

order to support use of the ARM Scalable Vector Extension (SVE).

This is an outline of the most important features and issues only and not

intended to be exhaustive.

This document does not aim to describe the SVE architecture or programmer's

model.  To aid understanding, a minimal description of relevant programmer's

model features for SVE is included in Appendix A.

1.  General

-----------

* SVE registers Z0..Z31, P0..P15 and FFR and the current vector length VL, are

  tracked per-thread.

* The presence of SVE is reported to userspace via HWCAP_SVE in the aux vector

  AT_HWCAP entry.  Presence of this flag implies the presence of the SVE

  instructions and registers, and the Linux-specific system interfaces

  described in this document.  SVE is reported in /proc/cpuinfo as "sve".

* Support for the execution of SVE instructions in userspace can also be

  detected by reading the CPU ID register ID_AA64PFR0_EL1 using an MRS

  instruction, and checking that the value of the SVE field is nonzero. [3]

  It does not guarantee the presence of the system interfaces described in the

  following sections: software that needs to verify that those interfaces are

  present must check for HWCAP_SVE instead.

* Debuggers should restrict themselves to interacting with the target via the

  NT_ARM_SVE regset.  The recommended way of detecting support for this regset

  is to connect to a target process first and then attempt a

  ptrace(PTRACE_GETREGSET, pid, NT_ARM_SVE, &iov).

2.  Vector length terminology

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

The size of an SVE vector (Z) register is referred to as the "vector length".

To avoid confusion about the units used to express vector length, the kernel

adopts the following conventions:

* Vector length (VL) = size of a Z-register in bytes

* Vector quadwords (VQ) = size of a Z-register in units of 128 bits

(So, VL = 16 * VQ.)

The VQ convention is used where the underlying granularity is important, such

as in data structure definitions.  In most other situations, the VL convention

is used.  This is consistent with the meaning of the "VL" pseudo-register in

the SVE instruction set architecture.

3.  System call behaviour

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

* On syscall, V0..V31 are preserved (as without SVE).  Thus, bits [127:0] of

  Z0..Z31 are preserved.  All other bits of Z0..Z31, and all of P0..P15 and FFR

  become unspecified on return from a syscall.

* The SVE registers are not used to pass arguments to or receive results from

  any syscall.

* In practice the affected registers/bits will be preserved or will be replaced

  with zeros on return from a syscall, but userspace should not make

  assumptions about this.  The kernel behaviour may vary on a case-by-case

  basis.

* All other SVE state of a thread, including the currently configured vector

  length, the state of the PR_SVE_VL_INHERIT flag, and the deferred vector

  length (if any), is preserved across all syscalls, subject to the specific

  exceptions for execve() described in section 6.

  In particular, on return from a fork() or clone(), the parent and new child

  process or thread share identical SVE configuration, matching that of the

  parent before the call.

4.  Signal handling

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

* A new signal frame record sve_context encodes the SVE registers on signal

  delivery. [1]

* This record is supplementary to fpsimd_context.  The FPSR and FPCR registers

  are only present in fpsimd_context.  For convenience, the content of V0..V31

  is duplicated between sve_context and fpsimd_context.

* The signal frame record for SVE always contains basic metadata, in particular

  the thread's vector length (in sve_context.vl).

* The SVE registers may or may not be included in the record, depending on

  whether the registers are live for the thread.  The registers are present if

  and only if:

  sve_context.head.size >= SVE_SIG_CONTEXT_SIZE(sve_vq_from_vl(sve_context.vl)).

* If the registers are present, the remainder of the record has a vl-dependent

  size and layout.  Macros SVE_SIG_* are defined [1] to facilitate access to

  the members.

* If the SVE context is too big to fit in sigcontext.__reserved[], then extra

  space is allocated on the stack, an extra_context record is written in

  __reserved[] referencing this space.  sve_context is then written in the

  extra space.  Refer to [1] for further details about this mechanism.

5.  Signal return

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

When returning from a signal handler:

* If there is no sve_context record in the signal frame, or if the record is

  present but contains no register data as desribed in the previous section,

  then the SVE registers/bits become non-live and take unspecified values.

* If sve_context is present in the signal frame and contains full register

  data, the SVE registers become live and are populated with the specified

  data.  However, for backward compatibility reasons, bits [127:0] of Z0..Z31

  are always restored from the corresponding members of fpsimd_context.vregs[]

  and not from sve_context.  The remaining bits are restored from sve_context.

* Inclusion of fpsimd_context in the signal frame remains mandatory,

  irrespective of whether sve_context is present or not.

* The vector length cannot be changed via signal return.  If sve_context.vl in

  the signal frame does not match the current vector length, the signal return

  attempt is treated as illegal, resulting in a forced SIGSEGV.

6.  prctl extensions

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

Some new prctl() calls are added to allow programs to manage the SVE vector

length:

prctl(PR_SVE_SET_VL, unsigned long arg)

    Sets the vector length of the calling thread and related flags, where

    arg == vl | flags.  Other threads of the calling process are unaffected.

    vl is the desired vector length, where sve_vl_valid(vl) must be true.

    flags:

	PR_SVE_SET_VL_INHERIT

	    Inherit the current vector length across execve().  Otherwise, the

	    vector length is reset to the system default at execve().  (See

	    Section 9.)

	PR_SVE_SET_VL_ONEXEC

	    Defer the requested vector length change until the next execve()

	    performed by this thread.

	    The effect is equivalent to implicit exceution of the following

	    call immediately after the next execve() (if any) by the thread:

		prctl(PR_SVE_SET_VL, arg & ~PR_SVE_SET_VL_ONEXEC)

	    This allows launching of a new program with a different vector

	    length, while avoiding runtime side effects in the caller.

	    Without PR_SVE_SET_VL_ONEXEC, the requested change takes effect

	    immediately.

    Return value: a nonnegative on success, or a negative value on error:

	EINVAL: SVE not supported, invalid vector length requested, or

	    invalid flags.

    On success:

    * Either the calling thread's vector length or the deferred vector length

      to be applied at the next execve() by the thread (dependent on whether

      PR_SVE_SET_VL_ONEXEC is present in arg), is set to the largest value

      supported by the system that is less than or equal to vl.  If vl ==

      SVE_VL_MAX, the value set will be the largest value supported by the

      system.

    * Any previously outstanding deferred vector length change in the calling

      thread is cancelled.

    * The returned value describes the resulting configuration, encoded as for

      PR_SVE_GET_VL.  The vector length reported in this value is the new

      current vector length for this thread if PR_SVE_SET_VL_ONEXEC was not

      present in arg; otherwise, the reported vector length is the deferred

      vector length that will be applied at the next execve() by the calling

      thread.

    * Changing the vector length causes all of P0..P15, FFR and all bits of

      Z0..V31 except for Z0 bits [127:0] .. Z31 bits [127:0] to become

      unspecified.  Calling PR_SVE_SET_VL with vl equal to the thread's current

      vector length, or calling PR_SVE_SET_VL with the PR_SVE_SET_VL_ONEXEC

      flag, does not constitute a change to the vector length for this purpose.

prctl(PR_SVE_GET_VL)

    Gets the vector length of the calling thread.

    The following flag may be OR-ed into the result:

	PR_SVE_SET_VL_INHERIT

	    Vector length will be inherited across execve().

    There is no way to determine whether there is an outstanding deferred

    vector length change (which would only normally be the case between a

    fork() or vfork() and the corresponding execve() in typical use).

    To extract the vector length from the result, and it with

    PR_SVE_VL_LEN_MASK.

    Return value: a nonnegative value on success, or a negative value on error:

	EINVAL: SVE not supported.

7.  ptrace extensions

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

* A new regset NT_ARM_SVE is defined for use with PTRACE_GETREGSET and

  PTRACE_SETREGSET.

  Refer to [2] for definitions.

The regset data starts with struct user_sve_header, containing:

    size

	Size of the complete regset, in bytes.

	This depends on vl and possibly on other things in the future.

	If a call to PTRACE_GETREGSET requests less data than the value of

	size, the caller can allocate a larger buffer and retry in order to

	read the complete regset.

    max_size

	Maximum size in bytes that the regset can grow to for the target

	thread.  The regset won't grow bigger than this even if the target

	thread changes its vector length etc.

    vl

	Target thread's current vector length, in bytes.

    max_vl

	Maximum possible vector length for the target thread.

    flags

	either

	    SVE_PT_REGS_FPSIMD

		SVE registers are not live (GETREGSET) or are to be made

		non-live (SETREGSET).

		The payload is of type struct user_fpsimd_state, with the same

		meaning as for NT_PRFPREG, starting at offset

		SVE_PT_FPSIMD_OFFSET from the start of user_sve_header.

		Extra data might be appended in the future: the size of the

		payload should be obtained using SVE_PT_FPSIMD_SIZE(vq, flags).

		vq should be obtained using sve_vq_from_vl(vl).

		or

	    SVE_PT_REGS_SVE

		SVE registers are live (GETREGSET) or are to be made live

		(SETREGSET).

		The payload contains the SVE register data, starting at offset

		SVE_PT_SVE_OFFSET from the start of user_sve_header, and with

		size SVE_PT_SVE_SIZE(vq, flags);

	... OR-ed with zero or more of the following flags, which have the same

	meaning and behaviour as the corresponding PR_SET_VL_* flags:

	    SVE_PT_VL_INHERIT

	    SVE_PT_VL_ONEXEC (SETREGSET only).

* The effects of changing the vector length and/or flags are equivalent to

  those documented for PR_SVE_SET_VL.

  The caller must make a further GETREGSET call if it needs to know what VL is

  actually set by SETREGSET, unless is it known in advance that the requested

  VL is supported.

* In the SVE_PT_REGS_SVE case, the size and layout of the payload depends on

  the header fields.  The SVE_PT_SVE_*() macros are provided to facilitate

  access to the members.

* In either case, for SETREGSET it is permissible to omit the payload, in which

  case only the vector length and flags are changed (along with any

  consequences of those changes).

* For SETREGSET, if an SVE_PT_REGS_SVE payload is present and the

  requested VL is not supported, the effect will be the same as if the

  payload were omitted, except that an EIO error is reported.  No

  attempt is made to translate the payload data to the correct layout

  for the vector length actually set.  The thread's FPSIMD state is

  preserved, but the remaining bits of the SVE registers become

  unspecified.  It is up to the caller to translate the payload layout

  for the actual VL and retry.

* The effect of writing a partial, incomplete payload is unspecified.

8.  ELF coredump extensions

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

* A NT_ARM_SVE note will be added to each coredump for each thread of the

  dumped process.  The contents will be equivalent to the data that would have

  been read if a PTRACE_GETREGSET of NT_ARM_SVE were executed for each thread

  when the coredump was generated.

9.  System runtime configuration

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

* To mitigate the ABI impact of expansion of the signal frame, a policy

  mechanism is provided for administrators, distro maintainers and developers

  to set the default vector length for userspace processes:

/proc/sys/abi/sve_default_vector_length

    Writing the text representation of an integer to this file sets the system

    default vector length to the specified value, unless the value is greater

    than the maximum vector length supported by the system in which case the

    default vector length is set to that maximum.

    The result can be determined by reopening the file and reading its

    contents.

    At boot, the default vector length is initially set to 64 or the maximum

    supported vector length, whichever is smaller.  This determines the initial

    vector length of the init process (PID 1).

    Reading this file returns the current system default vector length.

* At every execve() call, the new vector length of the new process is set to

  the system default vector length, unless

    * PR_SVE_SET_VL_INHERIT (or equivalently SVE_PT_VL_INHERIT) is set for the

      calling thread, or

    * a deferred vector length change is pending, established via the

      PR_SVE_SET_VL_ONEXEC flag (or SVE_PT_VL_ONEXEC).

* Modifying the system default vector length does not affect the vector length

  of any existing process or thread that does not make an execve() call.

Appendix A.  SVE programmer's model (informative)

=================================================

This section provides a minimal description of the additions made by SVE to the

ARMv8-A programmer's model that are relevant to this document.

Note: This section is for information only and not intended to be complete or

to replace any architectural specification.

A.1.  Registers

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

In A64 state, SVE adds the following:

* 32 8VL-bit vector registers Z0..Z31

  For each Zn, Zn bits [127:0] alias the ARMv8-A vector register Vn.

  A register write using a Vn register name zeros all bits of the corresponding

  Zn except for bits [127:0].

* 16 VL-bit predicate registers P0..P15

* 1 VL-bit special-purpose predicate register FFR (the "first-fault register")

* a VL "pseudo-register" that determines the size of each vector register

  The SVE instruction set architecture provides no way to write VL directly.

  Instead, it can be modified only by EL1 and above, by writing appropriate

  system registers.

* The value of VL can be configured at runtime by EL1 and above:

  16 <= VL <= VLmax, where VL must be a multiple of 16.

* The maximum vector length is determined by the hardware:

  16 <= VLmax <= 256.

  (The SVE architecture specifies 256, but permits future architecture

  revisions to raise this limit.)

* FPSR and FPCR are retained from ARMv8-A, and interact with SVE floating-point

  operations in a similar way to the way in which they interact with ARMv8

  floating-point operations.

         8VL-1                       128               0  bit index

        +----          ////            -----------------+

     Z0 |                               :       V0      |

      :                                          :

     Z7 |                               :       V7      |

     Z8 |                               :     * V8      |

      :                                       :  :

    Z15 |                               :     *V15      |

    Z16 |                               :      V16      |

      :                                          :

    Z31 |                               :      V31      |

        +----          ////            -----------------+

                                                 31    0

         VL-1                  0                +-------+

        +----       ////      --+          FPSR |       |

     P0 |                       |               +-------+

      : |                       |         *FPCR |       |

    P15 |                       |               +-------+

        +----       ////      --+

    FFR |                       |               +-----+

        +----       ////      --+            VL |     |

                                                +-----+

(*) callee-save:

    This only applies to bits [63:0] of Z-/V-registers.

    FPCR contains callee-save and caller-save bits.  See [4] for details.

A.2.  Procedure call standard

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

The ARMv8-A base procedure call standard is extended as follows with respect to

the additional SVE register state:

* All SVE register bits that are not shared with FP/SIMD are caller-save.

* Z8 bits [63:0] .. Z15 bits [63:0] are callee-save.

  This follows from the way these bits are mapped to V8..V15, which are caller-

  save in the base procedure call standard.

Appendix B.  ARMv8-A FP/SIMD programmer's model

===============================================

Note: This section is for information only and not intended to be complete or

to replace any architectural specification.

Refer to [4] for for more information.

ARMv8-A defines the following floating-point / SIMD register state:

* 32 128-bit vector registers V0..V31

* 2 32-bit status/control registers FPSR, FPCR

         127           0  bit index

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

     V0 |               |

      : :               :

     V7 |               |

   * V8 |               |

   :  : :               :

   *V15 |               |

    V16 |               |

      : :               :

    V31 |               |

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

                 31    0

                +-------+

           FPSR |       |

                +-------+

          *FPCR |       |

                +-------+

(*) callee-save:

    This only applies to bits [63:0] of V-registers.

    FPCR contains a mixture of callee-save and caller-save bits.

References

==========

[1] arch/arm64/include/uapi/asm/sigcontext.h

    AArch64 Linux signal ABI definitions

[2] arch/arm64/include/uapi/asm/ptrace.h

    AArch64 Linux ptrace ABI definitions

[3] linux/Documentation/arm64/cpu-feature-registers.txt

[4] ARM IHI0055C

    http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055c/IHI0055C_beta_aapcs64.pdf

    http://infocenter.arm.com/help/topic/com.arm.doc.subset.swdev.abi/index.html

    Procedure Call Standard for the ARM 64-bit Architecture (AArch64)

 * The SVE architecture leaves space for future expansion of the

 * vector length beyond its initial architectural limit of 2048 bits

 * (16 quadwords).

 *

 * See linux/Documentation/arm64/sve.txt for a description of the VL/VQ

 * terminology.

 */

#define SVE_VQ_BYTES		16	/* number of bytes per quadword */