Detailed Introduction to ARMv8/ARMv9 Page Table Attributes

1 Stage 1 Page Table Attributes

(Attribute fields in stage 1 VMSAv8-64 Block and Page descriptors)

PBHA, bits[62:59]: for FEAT_HPDS2
XN or UXN, bit[54]: Execute-never or Unprivileged execute-never
PXN, bit[53]: Privileged execute-never
Contiguous, bit[52]: The translation table entry is contiguous and can exist in a TLB Entry
DBM, bit[51]: Dirty Bit Modifier
GP, bit[50]: for FEAT_BTI
nT, bit[16]: for FEAT_BBM
nG, bit[11]: Whether the translation cached in TLB uses ASID identification
AF, bit[10]: Access flag, AF=0 means that the first access to this page will set this flag to 1, indicating the first access
SH, bits[9:8]: shareable attribute
AP,[2:1], bits[7:6]: Data Access Permissions bits,
NS, bit[5]: Non-secure bit
AttrIndx[2:0], bits[4:2]

2 Stage 2 Page Table Attributes

(Attribute fields in stage 2 VMSAv8-64 Block and Page descriptors)

PBHA[3:1], bits[62:60]: for FEAT_HPDS2
PBHA[0], bit[59]: for FEAT_HPDS2
XN[1:0], bits[54:53]: Execute-never
Contiguous, bit[52]: The translation table entry is contiguous and can exist in a TLB Entry
DBM, bit[51]: Dirty Bit Modifier
nT, bit[16]: for FEAT_BBM
FnXS, bit[11]: for FEAT_XS
AF, bit[10]: Access flag
SH, bits[9:8]: shareable attribute
S2AP, bits[7:6]: Stage 2 data Access Permissions
MemAttr, bits[5:2]

3 Detailed Introduction to Each Flag

3.1 MemAttr

3.2 NS

Non-secure bit indicates whether the converted physical address is secure or non-secure. In a dual-system environment with REE (Linux) and TEE (OP-TEE), both systems’ MMUs can be enabled simultaneously. Different page tables are used for secure and non-secure. The secure page table can map non-secure memory, while the non-secure page table cannot map secure memory; otherwise, an error will occur during conversion.

3.3 AP

Data access permissions

3.4 SH

Shareable attribute

3.5 AF

Access flag, when AF=0, the first access to this page will set this flag to 1, indicating the first access

3.6 nG

For EL0/EL1 virtual address space, the nG bit in the Page Descriptor attribute field marks the translation as Global (G) or non-Global (nG). For example, kernel mappings are Global (G) translations, while application mappings are non-Global translations. Global translations apply to any currently running application. Non-global translations apply only to specific applications.

Non-global mappings are tagged with ASID in the TLB. During TLB lookup, the ASID in the TLB entry is compared with the currently selected ASID. If they do not match, the TLB entry is not used. The following image shows global mappings without ASID tags in kernel space and non-global mappings with ASID tags in user space.

3.7 DBM

TODO

3.8 Contiguous