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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-11 08:27:49 +0000
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+===================================================
+Scalable Matrix Extension support for AArch64 Linux
+===================================================
+
+This document outlines briefly the interface provided to userspace by Linux in
+order to support use of the ARM Scalable Matrix Extension (SME).
+
+This is an outline of the most important features and issues only and not
+intended to be exhaustive. It should be read in conjunction with the SVE
+documentation in sve.rst which provides details on the Streaming SVE mode
+included in SME.
+
+This document does not aim to describe the SME architecture or programmer's
+model. To aid understanding, a minimal description of relevant programmer's
+model features for SME is included in Appendix A.
+
+
+1. General
+-----------
+
+* PSTATE.SM, PSTATE.ZA, the streaming mode vector length, the ZA and (when
+ present) ZTn register state and TPIDR2_EL0 are tracked per thread.
+
+* The presence of SME is reported to userspace via HWCAP2_SME in the aux vector
+ AT_HWCAP2 entry. Presence of this flag implies the presence of the SME
+ instructions and registers, and the Linux-specific system interfaces
+ described in this document. SME is reported in /proc/cpuinfo as "sme".
+
+* The presence of SME2 is reported to userspace via HWCAP2_SME2 in the
+ aux vector AT_HWCAP2 entry. Presence of this flag implies the presence of
+ the SME2 instructions and ZT0, and the Linux-specific system interfaces
+ described in this document. SME2 is reported in /proc/cpuinfo as "sme2".
+
+* Support for the execution of SME instructions in userspace can also be
+ detected by reading the CPU ID register ID_AA64PFR1_EL1 using an MRS
+ instruction, and checking that the value of the SME 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 HWCAP2_SME instead.
+
+* There are a number of optional SME features, presence of these is reported
+ through AT_HWCAP2 through:
+
+ HWCAP2_SME_I16I64
+ HWCAP2_SME_F64F64
+ HWCAP2_SME_I8I32
+ HWCAP2_SME_F16F32
+ HWCAP2_SME_B16F32
+ HWCAP2_SME_F32F32
+ HWCAP2_SME_FA64
+ HWCAP2_SME2
+
+ This list may be extended over time as the SME architecture evolves.
+
+ These extensions are also reported via the CPU ID register ID_AA64SMFR0_EL1,
+ which userspace can read using an MRS instruction. See elf_hwcaps.txt and
+ cpu-feature-registers.txt for details.
+
+* Debuggers should restrict themselves to interacting with the target via the
+ NT_ARM_SVE, NT_ARM_SSVE, NT_ARM_ZA and NT_ARM_ZT regsets. The recommended
+ way of detecting support for these regsets is to connect to a target process
+ first and then attempt a
+
+ ptrace(PTRACE_GETREGSET, pid, NT_ARM_<regset>, &iov).
+
+* Whenever ZA register values are exchanged in memory between userspace and
+ the kernel, the register value is encoded in memory as a series of horizontal
+ vectors from 0 to VL/8-1 stored in the same endianness invariant format as is
+ used for SVE vectors.
+
+* On thread creation TPIDR2_EL0 is preserved unless CLONE_SETTLS is specified,
+ in which case it is set to 0.
+
+2. Vector lengths
+------------------
+
+SME defines a second vector length similar to the SVE vector length which is
+controls the size of the streaming mode SVE vectors and the ZA matrix array.
+The ZA matrix is square with each side having as many bytes as a streaming
+mode SVE vector.
+
+
+3. Sharing of streaming and non-streaming mode SVE state
+---------------------------------------------------------
+
+It is implementation defined which if any parts of the SVE state are shared
+between streaming and non-streaming modes. When switching between modes
+via software interfaces such as ptrace if no register content is provided as
+part of switching no state will be assumed to be shared and everything will
+be zeroed.
+
+
+4. System call behaviour
+-------------------------
+
+* On syscall PSTATE.ZA is preserved, if PSTATE.ZA==1 then the contents of the
+ ZA matrix and ZTn (if present) are preserved.
+
+* On syscall PSTATE.SM will be cleared and the SVE registers will be handled
+ as per the standard SVE ABI.
+
+* None of the SVE registers, ZA or ZTn are used to pass arguments to
+ or receive results from any syscall.
+
+* On process creation (eg, clone()) the newly created process will have
+ PSTATE.SM cleared.
+
+* All other SME state of a thread, including the currently configured vector
+ length, the state of the PR_SME_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.
+
+
+5. Signal handling
+-------------------
+
+* Signal handlers are invoked with streaming mode and ZA disabled.
+
+* A new signal frame record TPIDR2_MAGIC is added formatted as a struct
+ tpidr2_context to allow access to TPIDR2_EL0 from signal handlers.
+
+* A new signal frame record za_context encodes the ZA register contents on
+ signal delivery. [1]
+
+* The signal frame record for ZA always contains basic metadata, in particular
+ the thread's vector length (in za_context.vl).
+
+* The ZA matrix may or may not be included in the record, depending on
+ the value of PSTATE.ZA. The registers are present if and only if:
+ za_context.head.size >= ZA_SIG_CONTEXT_SIZE(sve_vq_from_vl(za_context.vl))
+ in which case PSTATE.ZA == 1.
+
+* If matrix data is present, the remainder of the record has a vl-dependent
+ size and layout. Macros ZA_SIG_* are defined [1] to facilitate access to
+ them.
+
+* The matrix is stored as a series of horizontal vectors in the same format as
+ is used for SVE vectors.
+
+* If the ZA 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. za_context is then written in the
+ extra space. Refer to [1] for further details about this mechanism.
+
+* If ZTn is supported and PSTATE.ZA==1 then a signal frame record for ZTn will
+ be generated.
+
+* The signal record for ZTn has magic ZT_MAGIC (0x5a544e01) and consists of a
+ standard signal frame header followed by a struct zt_context specifying
+ the number of ZTn registers supported by the system, then zt_context.nregs
+ blocks of 64 bytes of data per register.
+
+
+5. Signal return
+-----------------
+
+When returning from a signal handler:
+
+* If there is no za_context record in the signal frame, or if the record is
+ present but contains no register data as described in the previous section,
+ then ZA is disabled.
+
+* If za_context is present in the signal frame and contains matrix data then
+ PSTATE.ZA is set to 1 and ZA is populated with the specified data.
+
+* The vector length cannot be changed via signal return. If za_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.
+
+* If ZTn is not supported or PSTATE.ZA==0 then it is illegal to have a
+ signal frame record for ZTn, resulting in a forced SIGSEGV.
+
+
+6. prctl extensions
+--------------------
+
+Some new prctl() calls are added to allow programs to manage the SME vector
+length:
+
+prctl(PR_SME_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_SME_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_SME_SET_VL_ONEXEC
+
+ Defer the requested vector length change until the next execve()
+ performed by this thread.
+
+ The effect is equivalent to implicit execution of the following
+ call immediately after the next execve() (if any) by the thread:
+
+ prctl(PR_SME_SET_VL, arg & ~PR_SME_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_SME_SET_VL_ONEXEC, the requested change takes effect
+ immediately.
+
+
+ Return value: a nonnegative on success, or a negative value on error:
+ EINVAL: SME 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_SME_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_SME_GET_VL. The vector length reported in this value is the new
+ current vector length for this thread if PR_SME_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 ZA, ZTn, P0..P15, FFR and all
+ bits of Z0..Z31 except for Z0 bits [127:0] .. Z31 bits [127:0] to become
+ unspecified, including both streaming and non-streaming SVE state.
+ Calling PR_SME_SET_VL with vl equal to the thread's current vector
+ length, or calling PR_SME_SET_VL with the PR_SVE_SET_VL_ONEXEC flag,
+ does not constitute a change to the vector length for this purpose.
+
+ * Changing the vector length causes PSTATE.ZA and PSTATE.SM to be cleared.
+ Calling PR_SME_SET_VL with vl equal to the thread's current vector
+ length, or calling PR_SME_SET_VL with the PR_SVE_SET_VL_ONEXEC flag,
+ does not constitute a change to the vector length for this purpose.
+
+
+prctl(PR_SME_GET_VL)
+
+ Gets the vector length of the calling thread.
+
+ The following flag may be OR-ed into the result:
+
+ PR_SME_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, bitwise and it with
+ PR_SME_VL_LEN_MASK.
+
+ Return value: a nonnegative value on success, or a negative value on error:
+ EINVAL: SME not supported.
+
+
+7. ptrace extensions
+---------------------
+
+* A new regset NT_ARM_SSVE is defined for access to streaming mode SVE
+ state via PTRACE_GETREGSET and PTRACE_SETREGSET, this is documented in
+ sve.rst.
+
+* A new regset NT_ARM_ZA is defined for ZA state for access to ZA state via
+ PTRACE_GETREGSET and PTRACE_SETREGSET.
+
+ Refer to [2] for definitions.
+
+The regset data starts with struct user_za_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 streaming vector length, in bytes.
+
+ max_vl
+
+ Maximum possible streaming vector length for the target thread.
+
+ flags
+
+ Zero or more of the following flags, which have the same
+ meaning and behaviour as the corresponding PR_SET_VL_* flags:
+
+ SME_PT_VL_INHERIT
+
+ SME_PT_VL_ONEXEC (SETREGSET only).
+
+* The effects of changing the vector length and/or flags are equivalent to
+ those documented for PR_SME_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.
+
+* The size and layout of the payload depends on the header fields. The
+ ZA_PT_ZA*() macros are provided to facilitate access to the data.
+
+* In either case, for SETREGSET it is permissible to omit the payload, in which
+ case the vector length and flags are changed and PSTATE.ZA is set to 0
+ (along with any consequences of those changes). If a payload is provided
+ then PSTATE.ZA will be set to 1.
+
+* For SETREGSET, if 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. 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.
+
+* A new regset NT_ARM_ZT is defined for access to ZTn state via
+ PTRACE_GETREGSET and PTRACE_SETREGSET.
+
+* The NT_ARM_ZT regset consists of a single 512 bit register.
+
+* When PSTATE.ZA==0 reads of NT_ARM_ZT will report all bits of ZTn as 0.
+
+* Writes to NT_ARM_ZT will set PSTATE.ZA to 1.
+
+
+8. ELF coredump extensions
+---------------------------
+
+* NT_ARM_SSVE notes 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 the corresponding
+ type were executed for each thread when the coredump was generated.
+
+* A NT_ARM_ZA 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_ZA were executed for each thread
+ when the coredump was generated.
+
+* A NT_ARM_ZT 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_ZT were executed for each thread
+ when the coredump was generated.
+
+* The NT_ARM_TLS note will be extended to two registers, the second register
+ will contain TPIDR2_EL0 on systems that support SME and will be read as
+ zero with writes ignored otherwise.
+
+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/sme_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 32 or the maximum
+ supported vector length, whichever is smaller and supported. 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_SME_VL_INHERIT (or equivalently SME_PT_VL_INHERIT) is set for the
+ calling thread, or
+
+ * a deferred vector length change is pending, established via the
+ PR_SME_SET_VL_ONEXEC flag (or SME_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. SME programmer's model (informative)
+=================================================
+
+This section provides a minimal description of the additions made by SME 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, SME adds the following:
+
+* A new mode, streaming mode, in which a subset of the normal FPSIMD and SVE
+ features are available. When supported EL0 software may enter and leave
+ streaming mode at any time.
+
+ For best system performance it is strongly encouraged for software to enable
+ streaming mode only when it is actively being used.
+
+* A new vector length controlling the size of ZA and the Z registers when in
+ streaming mode, separately to the vector length used for SVE when not in
+ streaming mode. There is no requirement that either the currently selected
+ vector length or the set of vector lengths supported for the two modes in
+ a given system have any relationship. The streaming mode vector length
+ is referred to as SVL.
+
+* A new ZA matrix register. This is a square matrix of SVLxSVL bits. Most
+ operations on ZA require that streaming mode be enabled but ZA can be
+ enabled without streaming mode in order to load, save and retain data.
+
+ For best system performance it is strongly encouraged for software to enable
+ ZA only when it is actively being used.
+
+* A new ZT0 register is introduced when SME2 is present. This is a 512 bit
+ register which is accessible when PSTATE.ZA is set, as ZA itself is.
+
+* Two new 1 bit fields in PSTATE which may be controlled via the SMSTART and
+ SMSTOP instructions or by access to the SVCR system register:
+
+ * PSTATE.ZA, if this is 1 then the ZA matrix is accessible and has valid
+ data while if it is 0 then ZA can not be accessed. When PSTATE.ZA is
+ changed from 0 to 1 all bits in ZA are cleared.
+
+ * PSTATE.SM, if this is 1 then the PE is in streaming mode. When the value
+ of PSTATE.SM is changed then it is implementation defined if the subset
+ of the floating point register bits valid in both modes may be retained.
+ Any other bits will be cleared.
+
+
+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] Documentation/arch/arm64/cpu-feature-registers.rst