src/mte_tag: HLV/HSV/MPRV support for opt-out based on PTE.MTAG for codepage (#82)

Guest might have set PTE.MTAG for code page and thus wouldn't be
expecting tag checks even if data pointer had PTE.MTAG set. On an exit
to hypervisor, hypervisor emulating this load/store must do emulation
similarly and thus HLVX* is updated to collect to MTAG bit in hstatus.
MTAG_I and use that during HLV/HSV.

Similar mechanisms when in M-mode and MPRV=1.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>

Author: deepak0414

src/mte_tag.adoc

@@ -380,11 +380,12 @@ tables is ignored.
 [[HYPERVISOR_LDST]]
 === Memory tagging and hypervisor memory accesses
 
-HLVX* instructions always generate unchecked loads. HLV*/HSV* instructions in
-HS and HU mode may generate checked accesses depending on effective privilege
-of VS or VU (as defined in privileged specification). Memory accesses generated
-from HLV*/HSV* instructions in HU mode may generate checked accesses when
-`hstatus.HU` is set.
+HLVX* instructions always generate unchecked loads.
+
+If `hstatus.MTAG_I` is set, then HLV*/HSV* instructions do not generate
+checked accesses. If `hstatus.MTAG_I` is clear, then HLV*/HSV* instructions
+in HS and HU mode may generate checked accesses depending on effective privilege
+of VS or VU (as defined in privileged specification).
 
 `xMT_MODE` (see <<MEM_TAG_EN>>) for HLV* and HSV* instructions is defined based
 on the effective privilege of VS or VU (as defined in privileged specification),
@@ -400,8 +401,24 @@ memory accesses based on rules specified by paging bit `MTAG` in first stage
 page tables (see <<TAGGED_PAGE>>).
 
 If HLV*/HSV* instructions result in a tag mismatch, software check exception is
-delivered to HS mode with tval = 4 and hstatus.GVA set. In case of tag mismatch,
-software check exception is always delivered synchronously.
+delivered to HS mode with tval = 4 and hstatus.GVA set.
+
+[[MPRV_LDST]]
+=== Memory tagging on loads/stores affected by Modify Privilege bit (MPRV)
+
+If MPRV=1 and `mstatus.MTAG_I` is set, then load and store instructions
+generate unchecked accesses. If MPRV=1 and `mstatus.MTAG_I` is clear, then
+load and store instructions may generate checked accesses depending on
+effective privilege. `xMT_MODE` is selected based on effective privilege (see
+<<MEM_TAG_EN>>).
+
+If page based virtual memory is enabled for effective privilege and memory
+tagging is enabled for effective privilege, then checked accesses are generated
+based on rules specified by paging bit `MTAG` in first stage page tables (see
+<<TAGGED_PAGE>>).
+
+In case of a tag mismatch, software check exception is delivered to M mode with
+tval = 4.
 
 [[MEMTAG_CSR_CTRL]]
 === CSR bits for memory tagging
@@ -574,6 +591,52 @@ When `MT_MODE` is `0b00`, the following rules apply to VS-mode:
 
 * Zimt instructions will revert to their behavior as defined by Zimop.
 
+==== Machine Status Register (`mstatus`)
+
+.Machine-mode status register (`mstatus`) for RV64
+[wavedrom, ,svg]
+....
+{reg: [
+  {bits:  1, name: 'WPRI'},
+  {bits:  1, name: 'SIE'},
+  {bits:  1, name: 'WPRI'},
+  {bits:  1, name: 'MIE'},
+  {bits:  1, name: 'WPRI'},
+  {bits:  1, name: 'SPIE'},
+  {bits:  1, name: 'UBE'},
+  {bits:  1, name: 'MPIE'},
+  {bits:  1, name: 'SPP'},
+  {bits:  2, name: 'VS[1:0]'},
+  {bits:  2, name: 'MPP[1:0]'},
+  {bits:  2, name: 'FS[1:0]'},
+  {bits:  2, name: 'XS[1:0]'},
+  {bits:  1, name: 'MPRV'},
+  {bits:  1, name: 'SUM'},
+  {bits:  1, name: 'MXR'},
+  {bits:  1, name: 'TVM'},
+  {bits:  1, name: 'TW'},
+  {bits:  1, name: 'TSR'},
+  {bits:  1, name: 'SPELP'},
+  {bits:  1, name: 'SDT'},
+  {bits:  7, name: 'WPRI'},
+  {bits:  2, name: 'UXL[1:0]'},
+  {bits:  2, name: 'SXL[1:0]'},
+  {bits:  1, name: 'SBE'},
+  {bits:  1, name: 'MBE'},
+  {bits:  1, name: 'GVA'},
+  {bits:  1, name: 'MPV'},
+  {bits:  1, name: 'MTAG_I'},
+  {bits:  1, name: 'MPELP'},
+  {bits:  1, name: 'MDT'},
+  {bits: 20, name: 'WPRI'},
+  {bits:  1, name: 'SD'},
+], config:{lanes: 4, hspace:1024}}
+....
+
+The Zimt extension adds `MTAG_I` bit to `mstatus`. When a trap is taken to
+M-mode and `mtinst` is written with nonzero value, then MTAG bit for code page
+of trapping instruction is deposited in `mstatus.MTAG_I`.
+
 ==== Hypervisor Status Register (`hstatus`)
 
 .Hypervisor status register (`hstatus`)
@@ -586,7 +649,8 @@ When `MT_MODE` is `0b00`, the following rules apply to VS-mode:
   {bits:  1, name: 'SPV'},
   {bits:  1, name: 'SPVP'},
   {bits:  1, name: 'HU'},
-  {bits:  2, name: 'WPRI'},
+  {bits:  1, name: 'MTAG_I'},
+  {bits:  1, name: 'WPRI'},
   {bits:  6, name: 'VGEIN'},
   {bits:  2, name: 'WPRI'},
   {bits:  1, name: 'VTVM'},
@@ -601,9 +665,12 @@ When `MT_MODE` is `0b00`, the following rules apply to VS-mode:
 ], config:{lanes: 4, hspace:1024}}
 ....
 
-The Zimt extension adds `VUMT_MODE` (bit 51:50) to `hstatus`. When the `HU`
-field is set, HLV*/HSV* may generate checked accesses in HU mode. `VUMT_MODE`
-selects the `xMT_MODE` if effective privilege mode is VU.
+The Zimt extension adds `VUMT_MODE` (bit 51:50) to `hstatus`. `VUMT_MODE`
+selects the `xMT_MODE` if execution environment is HU mode and effective
+privilege mode is VU. `hstatus.MTAG_I` bit emulates `MTAG` bit for
+instruction fetch from code page. When a trap is taken to HS mode and `htinst`
+is written with nonzero value, then VS-stage page table's MTAG bit on code page
+for trapping instruction is deposited in `hstatus.MTAG_I`.
 
 <<<