Guarded execution

Guarded execution mode is a lateral exception level next to EL1 and EL2 which uses the same pagetables but different permissions (see SPRR). These levels are called GL1 and GL2. It's enabled with bit 1 in S3_6_C15_1_2.

The instruction 0x00201420 is genter and switches from EL to GL and sets the PC to S3_6_C15_C8_1. 0x00201420 is gexit which returns to EL.

#define SYS_GXF_ENTER_EL1 sys_reg(3, 6, 15, 8, 1)

Guarded mode has a separate set of ELR, FAR, ESR, SPSR, VBAR and TPIDR registers just like EL1/2. Additionally the ASPSR register indicates if gexit should return to GL or EL.

#define SYS_TPIDR_GL1 sys_reg(3, 6, 15, 10, 1)
#define SYS_VBAR_GL1 sys_reg(3, 6, 15, 10, 2)
#define SYS_SPSR_GL1 sys_reg(3, 6, 15, 10, 3)
#define SYS_ASPSR_GL1 sys_reg(3, 6, 15, 10, 4)
#define SYS_ESR_GL1 sys_reg(3, 6, 15, 10, 5)
#define SYS_ELR_GL1 sys_reg(3, 6, 15, 10, 6)
#define SYS_FAR_GL1 sys_reg(3, 6, 15, 10, 7)

SPRR

SPRR takes the permission bits from pagetable entries and converts them into an attribute index similar to how MAIR works:

   3      2      1     0
 AP[1]  AP[0]   UXN   PXN

Note that UXN and PXN are flipped compared to APRR!

This is then used to index into a system register where each entry has four bits:

    3     2     1     0
  GL[1] GL[0] EL[1] EL[0]

GL/EL can be treated mostly separate but there are two exceptions where a specific GL permissions modifies what the two EL bits usually mean.

register value	EL page permissions	GL page permissions
0000	---	---
0001	r-x	---
0010	r--	---
0011	rw-	---
0100	---	r-x
0101	r-x	r-x
0110	r--	r-x
0111	---	r-x
1000	---	r--
1001	--x	r--
1010	r--	r--
1011	rw-	r--
1100	---	rw-
1101	r-x	rw-
1110	r--	rw-
1111	rw-	rw-

These four bits indicate the actual permissions when running in EL or GL mode. EL0 and EL1 have separate registers such that the permissions are decoupled.

bit 1 in S3_6_C15_C1_0 / SPRR_CONFIG_EL1 enables SPRR and access to new system registers.

S3_6_C15_1_5 is the permissions register for EL0 and S3_6_C15_1_6 is for EL1/GL1.

#define SYS_SPRR_CONFIG_EL1       sys_reg(3, 6, 15, 1, 0)
#define SPRR_CONFIG_EN            BIT(0)
#define SPRR_CONFIG_LOCK_CONFIG   BIT(1)
#define SPRR_CONFIG_LOCK_PERM_EL0 BIT(4)
#define SPRR_CONFIG_LOCK_PERM_EL1 BIT(5)

#define SYS_SPRR_PERM_EL0 sys_reg(3, 6, 15, 1, 5)
#define SYS_SPRR_PERM_EL1 sys_reg(3, 6, 15, 1, 6)