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Document a new SME support routine to query the current value of VG. #263

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May 29, 2024
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Document a new SME support routine to query the current value of VG. #263

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cd553ed
Document a new SME support routine to query the current value of VG.
kmclaughlin-arm May 23, 2024
851fa3d
Updated the change history table in aapcs64.rst.
kmclaughlin-arm May 28, 2024
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36 changes: 36 additions & 0 deletions aapcs64/aapcs64.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -254,6 +254,7 @@ changes to the content of the document for that release.
+------------+--------------------+------------------------------------------------------------------+
| | | - Change the status of the SME support from Alpha to Beta. |
| | | - Add soft-float PCS variant. |
| | | - Add the __arm_get_current_vg SME support routine. |
+------------+--------------------+------------------------------------------------------------------+

References
Expand Down Expand Up @@ -2077,6 +2078,9 @@ support routines:
``__arm_tpidr2_restore``
Provides a simple way of restoring lazily-saved ZA data.

``__arm_get_current_vg``
Provides a safe way to detect the current value of VG.

``__arm_sme_state``
^^^^^^^^^^^^^^^^^^^

Expand Down Expand Up @@ -2269,6 +2273,38 @@ a lazy save, with the subroutine having the following properties:
* The only memory modified by the subroutine (if any) is stack memory
below the incoming SP.

``__arm_get_current_vg``
^^^^^^^^^^^^^^^^^^^^^^^^

**(Beta)**

Platforms that support SME must provide a subroutine to query the current
value of VG, with the subroutine having the following properties:
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change to:

and the subroutine must have the following properties:

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addressed by #278


* The subroutine is called ``__arm_get_current_vg``.

* The subroutine has a `private-ZA`_ `streaming-compatible interface`_ with the
following properties:

* X1-X15, X19-X29 and SP are call-preserved.
* Z0-Z31 are call-preserved.
* P0-P15 are call-preserved.
* the subroutine `preserves ZA`_.

* The subroutine does not take any arguments.

* The subroutine returns an unsigned double word in X0.

* The subroutine behaves as follows:

* If the current thread has access to FEAT_SME and PSTATE.SM is 1, the
subroutine returns the value of the streaming VG in X0.

* Otherwise, if the current thread has access to FEAT_SVE, the subroutine
returns the value of VG in X0.

* Otherwise, the subroutine returns the value 0 in X0.

Pseudo-code examples
====================

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