arch/arm64/sme.rst

21 * PSTATE.SM, PSTATE.ZA, the streaming mode vector length, the ZA and (when
24 * The presence of SME is reported to userspace via HWCAP2_SME in the aux vector
30   aux vector AT_HWCAP2 entry.  Presence of this flag implies the presence of
75 2.  Vector lengths
78 SME defines a second vector length similar to the SVE vector length which
81 mode SVE vector.
109 * All other SME state of a thread, including the currently configured vector
110   length, the state of the PR_SME_VL_INHERIT flag, and the deferred vector
127   the thread's vector length (in za_context.vl).
167 * The vector length cannot be changed via signal return.  If za_context.vl in
168   the signal frame does not match the current vector length, the signal return
178 Some new prctl() calls are added to allow programs to manage the SME vector
183     Sets the vector length of the calling thread and related flags, where
186     vl is the desired vector length, where sve_vl_valid(vl) must be true.
192 	    Inherit the current vector length across execve().  Otherwise, the
193 	    vector length is reset to the system default at execve().  (See
198 	    Defer the requested vector length change until the next execve()
206 	    This allows launching of a new program with a different vector
214 	EINVAL: SME not supported, invalid vector length requested, or
220     * Either the calling thread's vector length or the deferred vector length
227     * Any previously outstanding deferred vector length change in the calling
231       PR_SME_GET_VL.  The vector length reported in this value is the new
232       current vector length for this thread if PR_SME_SET_VL_ONEXEC was not
233       present in arg; otherwise, the reported vector length is the deferred
234       vector length that will be applied at the next execve() by the calling
237     * Changing the vector length causes all of ZA, ZTn, P0..P15, FFR and all
240       Calling PR_SME_SET_VL with vl equal to the thread's current vector
242       does not constitute a change to the vector length for this purpose.
244     * Changing the vector length causes PSTATE.ZA and PSTATE.SM to be cleared.
245       Calling PR_SME_SET_VL with vl equal to the thread's current vector
247       does not constitute a change to the vector length for this purpose.
252     Gets the vector length of the calling thread.
258 	    Vector length will be inherited across execve().
261     vector length change (which would only normally be the case between a
264     To extract the vector length from the result, bitwise and it with
298 	thread changes its vector length etc.
302 	Target thread's current streaming vector length, in bytes.
306 	Maximum possible streaming vector length for the target thread.
317 * The effects of changing the vector length and/or flags are equivalent to
328   case the vector length and flags are changed and PSTATE.ZA is set to 0
335   for the vector length actually set.  It is up to the caller to translate the
377   to set the default vector length for userspace processes:
382     default vector length to the specified value rounded to a supported value
383     using the same rules as for setting vector length via PR_SME_SET_VL.
388     At boot, the default vector length is initially set to 32 or the maximum
389     supported vector length, whichever is smaller and supported.  This
390     determines the initial vector length of the init process (PID 1).
392     Reading this file returns the current system default vector length.
394 * At every execve() call, the new vector length of the new process is set to
395   the system default vector length, unless
400     * a deferred vector length change is pending, established via the
403 * Modifying the system default vector length does not affect the vector length
428 * A new vector length controlling the size of ZA and the Z registers when in
429   streaming mode, separately to the vector length used for SVE when not in
431   vector length or the set of vector lengths supported for the two modes in
432   a given system have any relationship.  The streaming mode vector length