arm64/sve: Add documentation

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

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@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
Signed-off-by: Will Deacon <will.deacon@arm.com>

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)

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 */