Added thread identifier (TID)

src/attributes.adoc

@@ -48,6 +48,8 @@ endif::[]
 :cheri_pte_ext_name:       Zcheri_pte
 // Extension for CHERI capabilities in vector registers
 :cheri_vectorcap_ext_name: Zcheri_vectorcap
+// Extension for thread identification
+:tid_ext_name: Zstid
 
 // Extension for supporting lr/sc.[bh]
 :lr_sc_bh_ext_name: Zabhlrsc

src/img/stidcreg.edn

@@ -0,0 +1,22 @@
+[bytefield]
+----
+(defattrs :plain [:plain {:font-family "M+ 1p Fallback" :font-size 21}])
+(def row-height 40)
+(def row-header-fn nil)
+(def left-margin 100)
+(def right-margin 100)
+(def boxes-per-row 34)
+(draw-column-headers {:height 20 :font-size 18 :labels (reverse ["0" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "MXLEN-1" "" ""])})
+
+(draw-box "Tag" {:span 1})
+(draw-box ""    {:span 1 :borders {}})
+
+(draw-box "stidc (Metadata)" {:span 32})
+
+(draw-box "" {:span 2 :borders {}})
+
+(draw-box "stidc (Address)"  {:span 32})
+
+(draw-box "" {:span 2 :borders {}})
+(draw-box "MXLEN" {:span 32 :borders {}})
+----

src/img/stidreg.edn

@@ -0,0 +1,14 @@
+[bytefield]
+----
+(defattrs :plain [:plain {:font-family "M+ 1p Fallback" :font-size 24}])
+(def row-height 40)
+(def row-header-fn nil)
+(def left-margin 100)
+(def right-margin 100)
+(def boxes-per-row 32)
+(draw-column-headers {:height 20 :font-size 18 :labels (reverse ["0" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "SXLEN-1"])})
+
+(draw-box "stid"  {:span 32})
+
+(draw-box "SXLEN" {:span 32 :borders {}})
+----

src/img/utidcreg.edn

@@ -0,0 +1,22 @@
+[bytefield]
+----
+(defattrs :plain [:plain {:font-family "M+ 1p Fallback" :font-size 21}])
+(def row-height 40)
+(def row-header-fn nil)
+(def left-margin 100)
+(def right-margin 100)
+(def boxes-per-row 34)
+(draw-column-headers {:height 20 :font-size 18 :labels (reverse ["0" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "MXLEN-1" "" ""])})
+
+(draw-box "Tag" {:span 1})
+(draw-box ""    {:span 1 :borders {}})
+
+(draw-box "utidc (Metadata)" {:span 32})
+
+(draw-box "" {:span 2 :borders {}})
+
+(draw-box "utidc (Address)"  {:span 32})
+
+(draw-box "" {:span 2 :borders {}})
+(draw-box "MXLEN" {:span 32 :borders {}})
+----

src/img/utidreg.edn

@@ -0,0 +1,14 @@
+[bytefield]
+----
+(defattrs :plain [:plain {:font-family "M+ 1p Fallback" :font-size 24}])
+(def row-height 40)
+(def row-header-fn nil)
+(def left-margin 100)
+(def right-margin 100)
+(def boxes-per-row 32)
+(draw-column-headers {:height 20 :font-size 18 :labels (reverse ["0" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "UXLEN-1"])})
+
+(draw-box "utid"  {:span 32})
+
+(draw-box "UXLEN" {:span 32 :borders {}})
+----

src/introduction.adoc

@@ -95,6 +95,8 @@ virtual-memory
 {cheri_vectorcap_ext_name}:: CHERI extension for the RISC-V Vector (V)
 extension. It adds support for storing CHERI capabilities in
 vector registers, intended for vectorised memory copying
+{tid_ext_name}:: Extension for supporting thread identifiers. This extension
+improves software compartmentalization on CHERI systems.
 
 CAUTION: The extension names are provisional and subject to change.
 

src/riscv-cheri.adoc

@@ -50,6 +50,8 @@ include::cheri-pte-ext.adoc[]
 
 include::riscv-legacy-integration.adoc[]
 
+include::tid-ext.adoc[]
+
 include::instructions.adoc[]
 
 include::tables.adoc[]

src/tid-ext.adoc

@@ -0,0 +1,107 @@
+== "Zstid Extension for Thread Identification
+
+{tid_ext_name} is an optional extension to the RISC-V base ISA.
+Implementations that support {cheri_base_ext_name} and {tid_ext_name}.
+define a variant of the CHERI ISA that allows for more efficient
+software compartmentalization of CHERI programs.
+
+=== Control and Status Registers (CSRs)
+
+{tid_ext_name} adds two new CSRs to implement a trusted thread
+identifier (TID) used in compartmentalization. These CSRs are listed in
+xref:tid-scsrnames-added[xrefstyle=short] and
+xref:tid-ucsrnames-added[xrefstyle=short].
+
+[[tid-scsrnames-added]]
+.Added supervisor-mode CSRs in {tid_ext_name}
+[%autowidth,float="center",align="center",cols="<,<,<,<,<",options="header"]
+|===
+|{tid_ext_name} CSR|Address|Prerequisites|Permissions|Description
+|<<stid>>|0x580|S-mode|SRW, <<asr_perm>>|Supervisor Thread Identifier
+|===
+
+[[tid-ucsrnames-added]]
+.Added user-mode CSRs in {tid_ext_name}
+[%autowidth,float="center",align="center",cols="<,<,<,<,<",options="header"]
+|===
+|{tid_ext_name} CSR|Address|Prerequisites|Permissions|Description
+|<<utid>>|0xC80|U-mode|URO|User Thread Identifier
+|===
+
+=== Supervisor-Level and Unprivileged CSRs
+
+[#stid,reftext="stid"]
+==== Supervisor Thread Identifier (stid)
+
+The <<stid>> register is an SXLEN-bit read-write register. It is used to
+identify the current thread. The reset value of this register is
+UNSPECIFIED.
+
+.Supervisor thread identifier register
+include::img/stidreg.edn[]
+
+
+[#utid,reftext="utid"]
+==== User Thread Identifier (utid)
+
+The <<utid>> register is an UXLEN-bit read-only register. It is a read-only
+copy of the <<stid>> register. The reset value of this register is
+UNSPECIFIED.
+
+.User thread identifier register
+include::img/utidreg.edn[]
+
+When {cheri_base_ext_name} is implemented, the {tid_ext_name} CSRs are
+extended as follows:
+
+[#stidc,reftext="stidc"]
+==== Supervisor Thread Identifier Capability (stidc)
+
+The <<stidc>> register is an CLEN-bit read-write capability register.
+It is the capability extension of the <<stid>> register.
+It is used to identify the current thread. On reset the tag of <<stidc>>
+will be set to 0 and the remainder of the data is UNSPECIFIED.
+
+.Supervisor thread identifier capability register
+include::img/stidcreg.edn[]
+
+
+[#utidc,reftext="utidc"]
+==== User Thread Identifier Capability (utidc)
+
+The <<utidc>> register is an CLEN-wide read-only capability register.
+It is the capability extension of the <<utid>> register. <<utidc>> is
+a read-only copy of the <<stidc>> register. On reset the tag of <<utidc>>
+will be set to 0 and the remainder of the data is UNSPECIFIED.
+
+.User thread identifier capability register
+include::img/utidcreg.edn[]
+
+[#CHERI_COMP,reftext="CHERI Compartmentalization]
+=== CHERI Compartmentalization
+
+This section describes how this specification enables support for
+compartmentalization for CHERI systems. Compartmentalization seeks
+to separate the privileges between different protection units, e.g.,
+two or more libraries. Code can be separated by sentries, which allow
+for giving out code capabilities to untrusted code where the untrusted
+code can only call the code capability, but not modify it. Sentries can
+be called from different threads and thus there needs to be a way of
+identifying the current thread. While identifying the current thread
+can be done by privileged code, e.g., the kernel, the implied performance
+overhead of this is not bearable for CHERI systems with many compartments.
+
+The RISC-V ABI includes a _thread pointer (tp)_ register, which is not
+usable for the purpose of reliably identifying the current thread because
+the tp register is a general purpose register and can be changed arbitrarily
+by untrusted code. Therefore, this specification offers two additional CSRs
+that facilitate a trusted source for the thread ID. The supervisor thread
+identifier (<<stid>>) register is readable and writeable with <<asr_perm>>
+from the supervisor mode allowing to change the thread ID on a context switch.
+The user thread identifier (<<utid>>) exposes the current value of <<stid>> as a
+read-only copy.
+
+This extension extends <<stid>> to its capability variant <<stidc>> and
+<<utid>> to its capability variant <<utidc>>. This presents software with the
+freedom to still use these registers with capabilities or leave the metadata
+untouched and only use the registers to storage integers.