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Specify behaviour for reserved permission encoding #53

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Feb 2, 2024
Merged

Specify behaviour for reserved permission encoding #53

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9 changes: 7 additions & 2 deletions src/cap-description.adoc
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Original file line number Diff line number Diff line change
Expand Up @@ -144,8 +144,13 @@ permission.
WARNING: #TODO: Confirm that we need a separate permissions format for 32-bit
and 64-bit.#

NOTE: Valid capabilities must not have the permissions field set to a reserved
value according to xref:cap_perms_encoding32[xrefstyle=short] when XLENMAX=32.
NOTE: When XLENMAX=32 there is a single reserved permission encoding (see
xref:cap_perms_encoding32[xrefstyle=short]). It is not possible for a tagged
capability to have this value since <<CANDPERM>> will never create it. It is
possible for untagged capabilities to have it. <<CGETPERM>> will interpret it
as if it were 0b0000 (no permissions). Future extensions may assign meaning to
the reserved bit pattern, in which case <<CGETPERM>> is allowed to report a
non-zero value.

[#section_cap_sdp]
==== Software-Defined Permissions (SDP)
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7 changes: 5 additions & 2 deletions src/insns/cgetperm_32bit.adoc
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Expand Up @@ -15,9 +15,12 @@ Encoding::
include::wavedrom/cgetperm.adoc[]

Description::
Converts the AP and SDP fields of capability `cs1` into a bit field; one bit
Converts the AP and SDP fields of capability `cs1` into a bit field; one bit
per permission, as shown below, and write the result to `rd`. A bit set to 1
in the bit field indicates that `cs1` grants the corresponding permission.
in the bit field indicates that `cs1` grants the corresponding permission.
+
If the AP field is a reserved value then all architectural permission bits in
`rd` are set to 0.

include::../img/candperms_bit_field.edn[]

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