[PATCH v4 00/28] ARM Scalable Vector Extension (SVE)

[PATCH v4 00/28] ARM Scalable Vector Extension (SVE)

[Dave Martin]

This series implements Linux kernel support for the ARM Scalable Vector
Extension (SVE). [1]  It supersedes the previous v3: see [3] for link.
See the individual patches for details of changes.

The patches apply on
git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git
for-next/core
fa3eb71d96f5 ("arm64: asm-bug: Renumber macro local labels to avoid
clashes")

For reviewer convenience, a git tree is available. [4]

To reduce spam, some people may not been copied on the entire series.
For those who did not receive the whole series, it can be found in the
linux-arm-kernel archive. [2]


*Note* The final two patches (27-28) of the series are still RFC and not
proposed for merging at present.  Before committing to this ABI it would
be good to get feedback on whether the approach makes sense and whether
it suitable for other architectures.  These two patches are not required
by the rest of the series and can be revised or merged later.


Support for use of SVE by KVM guests is not currently included.
Instead, such use will be trapped and reflected to the guest as
undefined instruction execution.  SVE is hidden from the view of the
CPU feature registers visible to guests, so that guests will not
expect it to work.


This series has been build- and boot-tested on the ARM FVP Base model
with and without the SVE plugin, and on ARM Juno r0.  Because there is
no hardware with SVE support yet, testing of the SVE functionality has
only been performed on the model.

Additional testing is under way.


Series summary:

 * Patches 1-5 contain some individual bits of preparatory spadework,
   which are indirectly related to SVE.

Dave Martin (5):
  regset: Add support for dynamically sized regsets
  arm64: KVM: Hide unsupported AArch64 CPU features from guests
  arm64: efi: Add missing Kconfig dependency on KERNEL_MODE_NEON
  arm64: Port deprecated instruction emulation to new sysctl interface
  arm64: fpsimd: Simplify uses of {set,clear}_ti_thread_flag()

   Non-trivial changes among these are:

   * Patch 1: updates the regset core code to handle regsets whose size
     is not fixed at compile time.  This avoids bloating coredumps even
     though the maximum theoretical SVE regset size is large.

   * Patch 2: extends KVM to modify the ARM architectural ID registers
     seen by guests, by trapping and emulating certain registers.  For
     SVE this is a temporary measure, but it may be useful for other
     architecture extensions.  This patch may also be built on in the
     future, since the only registers currently emulated are those
     required for hiding SVE.

 * Patches 6-10 add SVE-specific system register and structure layout
   definitions, and the low-level boot code and accessors needed for
   making use of SVE.

Dave Martin (5):
  arm64/sve: System register and exception syndrome definitions
  arm64/sve: Low-level SVE architectural state manipulation functions
  arm64/sve: Kconfig update and conditional compilation support
  arm64/sve: Signal frame and context structure definition
  arm64/sve: Low-level CPU setup

 * Patches 11-13 implement the core context management facilities to
   provide each user task with its own SVE register context, signal
   handling facilities, and sane programmer's model interoperation
   between SVE and FPSIMD.

Dave Martin (3):
  arm64/sve: Core task context handling
  arm64/sve: Support vector length resetting for new processes
  arm64/sve: Signal handling support

 * Patches 14 and 16 provide backend logic for detecting and making use
   of the different SVE vector lengths supported by the hardware.

 * Patch 15 moves around code in cpufeatures.c to fit.

Dave Martin (3):
  arm64/sve: Backend logic for setting the vector length
  arm64: cpufeature: Move sys_caps_initialised declarations
  arm64/sve: Probe SVE capabilities and usable vector lengths

 * Patches 17-18 update the kernel-mode NEON / EFI FPSIMD frameworks to
   interoperate correctly with SVE.

Dave Martin (2):
  arm64/sve: Preserve SVE registers around kernel-mode NEON use
  arm64/sve: Preserve SVE registers around EFI runtime service calls

 * Patches 19-21 implement the userspace frontend for managing SVE,
   comprising ptrace, some new arch-specific prctl() calls, and a new
   sysctl for init-time setup.

Dave Martin (3):
  arm64/sve: ptrace and ELF coredump support
  arm64/sve: Add prctl controls for userspace vector length management
  arm64/sve: Add sysctl to set the default vector length for new
    processes

 * Patches 22-24 provide stub KVM extensions for using KVM only on the
   host, while denying guest access.  (A future series will extend this
   with full support for SVE in guests.)

Dave Martin (3):
  arm64/sve: KVM: Prevent guests from using SVE
  arm64/sve: KVM: Treat guest SVE use as undefined instruction
    execution
  arm64/sve: KVM: Hide SVE from CPU features exposed to guests

And finally:

 * Patch 25 disengages the safety catch, enabling the kernel SVE runtime
   support and allowing userspace to use SVE.

Dave Martin (1):
  arm64/sve: Detect SVE and activate runtime support

 * Patch 26 adds some basic documentation.

Dave Martin (1):
  arm64/sve: Add documentation

 * Patches 27-28 (which may be considered RFC) propose a mechanism to
   report the maximum runtime signal frame size to userspace.

Dave Martin (2):
  arm64: signal: Report signal frame size to userspace via auxv
  arm64/sve: signal: Include SVE when computing AT_MINSIGSTKSZ


References:

[1] ARM Scalable Vector Extension
https://community.arm.com/groups/processors/blog/2016/08/22/technology-update-the-scalable-vector-extension-sve-for-the-armv8-a-architecture

[2] linux-arm-kernel October 2017 Archives by thread
http://lists.infradead.org/pipermail/linux-arm-kernel/2017-October/thread.html

[3] [PATCH v3 00/28] ARM Scalable Vector Extension (SVE)
http://lists.infradead.org/pipermail/linux-arm-kernel/2017-October/535717.html

[4] For review and testing only -- **do not pull**
    (This branch has review changelogs which should not form part of
    the final commits.)

    http://linux-arm.org/git?p=linux-dm.git;a=shortlog;h=refs/heads/sve/v4
    git://linux-arm.org/linux-dm.git sve/v4


Full series and diffstat:

Dave Martin (28):
  regset: Add support for dynamically sized regsets
  arm64: KVM: Hide unsupported AArch64 CPU features from guests
  arm64: efi: Add missing Kconfig dependency on KERNEL_MODE_NEON
  arm64: Port deprecated instruction emulation to new sysctl interface
  arm64: fpsimd: Simplify uses of {set,clear}_ti_thread_flag()
  arm64/sve: System register and exception syndrome definitions
  arm64/sve: Low-level SVE architectural state manipulation functions
  arm64/sve: Kconfig update and conditional compilation support
  arm64/sve: Signal frame and context structure definition
  arm64/sve: Low-level CPU setup
  arm64/sve: Core task context handling
  arm64/sve: Support vector length resetting for new processes
  arm64/sve: Signal handling support
  arm64/sve: Backend logic for setting the vector length
  arm64: cpufeature: Move sys_caps_initialised declarations
  arm64/sve: Probe SVE capabilities and usable vector lengths
  arm64/sve: Preserve SVE registers around kernel-mode NEON use
  arm64/sve: Preserve SVE registers around EFI runtime service calls
  arm64/sve: ptrace and ELF coredump support
  arm64/sve: Add prctl controls for userspace vector length management
  arm64/sve: Add sysctl to set the default vector length for new
    processes
  arm64/sve: KVM: Prevent guests from using SVE
  arm64/sve: KVM: Treat guest SVE use as undefined instruction execution
  arm64/sve: KVM: Hide SVE from CPU features exposed to guests
  arm64/sve: Detect SVE and activate runtime support
  arm64/sve: Add documentation
  arm64: signal: Report signal frame size to userspace via auxv
  arm64/sve: signal: Include SVE when computing AT_MINSIGSTKSZ

 Documentation/arm64/cpu-feature-registers.txt |   6 +-
 Documentation/arm64/elf_hwcaps.txt            |   4 +
 Documentation/arm64/sve.txt                   | 508 +++++++++++++++
 arch/arm/include/asm/kvm_host.h               |   3 +
 arch/arm64/Kconfig                            |  12 +
 arch/arm64/include/asm/cpu.h                  |   4 +
 arch/arm64/include/asm/cpucaps.h              |   3 +-
 arch/arm64/include/asm/cpufeature.h           |  42 ++
 arch/arm64/include/asm/elf.h                  |   5 +
 arch/arm64/include/asm/esr.h                  |   3 +-
 arch/arm64/include/asm/fpsimd.h               |  71 +-
 arch/arm64/include/asm/fpsimdmacros.h         | 148 +++++
 arch/arm64/include/asm/kvm_arm.h              |   5 +-
 arch/arm64/include/asm/kvm_host.h             |  11 +
 arch/arm64/include/asm/processor.h            |  10 +
 arch/arm64/include/asm/sysreg.h               |  24 +
 arch/arm64/include/asm/thread_info.h          |   5 +
 arch/arm64/include/asm/traps.h                |   2 +
 arch/arm64/include/uapi/asm/auxvec.h          |   3 +-
 arch/arm64/include/uapi/asm/hwcap.h           |   1 +
 arch/arm64/include/uapi/asm/ptrace.h          | 138 ++++
 arch/arm64/include/uapi/asm/sigcontext.h      | 120 +++-
 arch/arm64/kernel/armv8_deprecated.c          |  15 +-
 arch/arm64/kernel/cpufeature.c                |  99 ++-
 arch/arm64/kernel/cpuinfo.c                   |   7 +
 arch/arm64/kernel/entry-fpsimd.S              |  17 +
 arch/arm64/kernel/entry.S                     |  40 +-
 arch/arm64/kernel/fpsimd.c                    | 902 +++++++++++++++++++++++++-
 arch/arm64/kernel/head.S                      |  13 +-
 arch/arm64/kernel/process.c                   |  24 +
 arch/arm64/kernel/ptrace.c                    | 280 +++++++-
 arch/arm64/kernel/signal.c                    | 216 +++++-
 arch/arm64/kernel/signal32.c                  |   2 +-
 arch/arm64/kernel/traps.c                     |   7 +-
 arch/arm64/kvm/handle_exit.c                  |   8 +
 arch/arm64/kvm/hyp/switch.c                   |  12 +-
 arch/arm64/kvm/sys_regs.c                     | 292 +++++++--
 fs/binfmt_elf.c                               |   6 +-
 include/linux/regset.h                        |  67 +-
 include/uapi/linux/elf.h                      |   1 +
 include/uapi/linux/prctl.h                    |   9 +
 kernel/sys.c                                  |  12 +
 virt/kvm/arm/arm.c                            |   3 +
 43 files changed, 3014 insertions(+), 146 deletions(-)
 create mode 100644 Documentation/arm64/sve.txt

-- 
2.1.4

[PATCH v4 26/28] arm64/sve: Add documentation

[Dave Martin]

This patch adds basic documentation of the user/kernel interface
provided by the for SVE.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Cc: Alan Hayward <alan.hayward@arm.com>
Cc: Alex Bennée <alex.bennee@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc: linux-api@vger.kernel.org

---

Changes since v3
----------------

Changes requested by Catalin Marinas

 * Typo fix: SIG_SVE_ -> SVE_SIG_ in sigcontext macro names.
---
 Documentation/arm64/sve.txt              | 508 +++++++++++++++++++++++++++++++
 arch/arm64/include/uapi/asm/sigcontext.h |   3 +
 2 files changed, 511 insertions(+)
 create mode 100644 Documentation/arm64/sve.txt

diff --git a/Documentation/arm64/sve.txt b/Documentation/arm64/sve.txt
new file mode 100644
index 0000000..f128f73
--- /dev/null
+++ b/Documentation/arm64/sve.txt
@@ -0,0 +1,508 @@
+            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)
diff --git a/arch/arm64/include/uapi/asm/sigcontext.h b/arch/arm64/include/uapi/asm/sigcontext.h
index 7654a81..3c0b484 100644
--- a/arch/arm64/include/uapi/asm/sigcontext.h
+++ b/arch/arm64/include/uapi/asm/sigcontext.h
@@ -133,6 +133,9 @@ struct sve_context {
  * 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 */
 
-- 
2.1.4

Re: [PATCH v4 26/28] arm64/sve: Add documentation

[Catalin Marinas]

On Fri, Oct 27, 2017 at 11:51:08AM +0100, Dave P Martin wrote:
> This patch adds basic documentation of the user/kernel interface
> provided by the for SVE.
> 
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> Cc: Alan Hayward <alan.hayward@arm.com>
> Cc: Alex Bennée <alex.bennee@linaro.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Michael Kerrisk <mtk.manpages@gmail.com>
> Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>
> Cc: linux-api@vger.kernel.org

Acked-by: Catalin Marinas <catalin.marinas@arm.com>