[RFC PATCH v2 41/41] arm64/sve: Documentation: Add overview of the SVE userspace ABI
Dave Martin
Dave.Martin at arm.com
Wed Mar 22 07:51:11 PDT 2017
This patch adds initial documentation of the ABI provided by the
kernel to enable userspace software to make use of SVE.
Signed-off-by: Dave Martin <Dave.Martin at arm.com>
---
Documentation/arm64/sve.txt | 475 ++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 475 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..f82fbf8
--- /dev/null
+++ b/Documentation/arm64/sve.txt
@@ -0,0 +1,475 @@
+ Scalable Vector Extension support for AArch64 Linux
+ ===================================================
+
+Author: Dave Martin <Dave.Martin at arm.com>
+Date: 20 March 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. SVE is reported in /proc/cpuinfo as "sve".
+
+
+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.
+
+
+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 sve_context record is included if and only if the SVE registers are live
+ at the time of signal delivery.
+
+If the sve_context record is present in the signal frame:
+
+* sve_context.vl encodes the current vector length of the thread.
+
+* The remainder of the data has a vl-dependent size and layout. Macros
+ SIG_SVE_* 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, the SVE registers/bits
+ become non-live and unspecified. (In practice, this is likely to mean that
+ these bits become zero, but software should not make assumptions about this.)
+
+* If sve_context is present in the signal frame, the SVE registers become live
+ and are populated appropriately. 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 vl, unsigned long flags)
+
+ Sets the vector length of the calling thread.
+
+ flags:
+
+ PR_SVE_SET_VL_THREAD
+
+ Permit setting even if the calling process has multiple threads.
+ This is otherwise forbidden in order to prevent accidents.
+
+ Worker threads with different vector lengths can be created in this
+ way, within a single process.
+
+ General-purpose userspace code may not work correctly unless all
+ threads have the same vector length.
+
+ 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().
+ This allows launching of a new program with a different vector
+ length, while avoiding runtime side effects in the caller.
+
+ This also overrides the effect of PR_SVE_SET_VL_INHERIT for the
+ first execve().
+
+ Without PR_SVE_SET_VL_ONEXEC, any outstanding deferred vector
+ length change is cancelled.
+
+ Return value: a nonnegative on success, or a negative value on error:
+ EINVAL: SVE not supported, invalid vector length requested, or
+ invalid flags.
+
+ The returned value describes the resulting configuration, encoded as for
+ PR_SVE_GET_VL.
+
+ 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 does not constitute a change to the vector length for this
+ purpose.
+
+ If vl is greater than the maximum vector length supported by the system,
+ the vector length is set to the maximum supported vector length.
+
+
+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, use PR_SVE_GET_VL_LEN().
+
+ Return value: a nonnegative value on success, or a negative value on error:
+ EINVAL: SVE not supported.
+
+
+prctl(PR_GET_MINSIGSTKSZ)
+
+ Returns the minimum amount of stack space in bytes required for guaranteed
+ signal delivery.
+
+ The returned value is analogous to the POSIX macro MINSIGSTKSZ, except that
+ expansion of the signal frame is taken into account. This allows correct
+ allocation of stacks even for large vector lengths.
+
+ Return value: a nonnegative value on success, or a negative value on error:
+ EINVAL: Function not implemented.
+
+ If this call fails, the value provided for MINSIGSTKSZ by [1] can be
+ assumed.
+
+
+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_THREAD (SETREGSET only)
+
+ 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.
+
+* 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).
+
+ 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.
+
+ Each of these notes may be padded up to a larger size: if so, the surplus
+ data should be ignored when parsing the coredump.
+
+
+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/cpu/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 [3] 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 [3] 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] 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)
--
2.1.4
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