src/mte_tag: HLV/HSV support for opt-out based on PTE.MTAG for codepage

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.
VMTAG_FETCH and use that during HLV/HSV.

Signed-off-by: Deepak Gupta <[email protected]>

Author: deepak0414

src/mte_tag.adoc

@@ -392,11 +392,14 @@ 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. HLVX* instructions collect
+MTAG bit from VS-stage page table for code page of guest instruction and spill
+it in `hstatus.VMTAG_FETCH`.
+
+If `hstatus.VMTAG_FETCH` is set, then HLV*/HSV* instructions do not generate
+checked accesses. If `hstatus.VMTAG_FETCH` 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),
@@ -605,7 +608,7 @@ When `MT_MODE` is `0b00`, the following rules apply to VS-mode:
   {bits:  1, name: 'GVA'},
   {bits:  1, name: 'SPV'},
   {bits:  1, name: 'SPVP'},
-  {bits:  1, name: 'HU'},
+  {bits:  1, name: 'VMTAG_FETCH'},
   {bits:  2, name: 'WPRI'},
   {bits:  6, name: 'VGEIN'},
   {bits:  2, name: 'WPRI'},
@@ -621,9 +624,12 @@ 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.VMTAG_FETCH` bit emulates `MTAG` bit for
+instruction fetch from code page. HSX* instruction walks VS-stage page table
+to fetch instruction and additionally collects `MTAG` bit and deposits in the
+`hstatus.VMTAG_FETCH` bit.
 
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