SecureCells Architecture: Traps

Traps in SecureCells can originate from features in the base RISC-V architecture, or from additions made for SecureCells. This page documents the relevant trap behaviors.

Traps due to access control errors

Access control check failures raise traps with SecureCells. These traps mirror the behavior as with baseline RISC-V architecture. We describe these traps below, with the corresponding exception code for scause in brackets.

1. Load and store instructions raise “Load access fault” (0x4) and “Store/AMO” access fault (0x7), when the executing USID does not have the respective permission to the accessed address.
2. Instruction fetch from permissions raise a “Instruction Page Fault” (0xc) when the executing USID does not have execute permission for the instruction pointer.
3. Environment calls from userspace (ecall) raise the same trap number.

Additional traps introduced by SecureCells uses the range 0x18-0x1c for exception codes in `scause.

Inst	Cause	scause	stval
SCExcl	Illegal address	_ILL_ADDR	addr
SCExcl	Illegal permissions	_ILL_PERM	(type << 8) | perm type = 0 if perm not in RWX 1 if perm is zero 2 if SDcur has insufficient perms
SCExcl	Cell description invalid	_INV_CELL_STATE	0 if cell is invalid
SCGrant	Illegal address	_ILL_ADDR	addr
SCGrant	Target too high	_INV_SDID	SDtgt
SCGrant	Illegal permissions	_ILL_PERM	(type << 8) | perm type = 0 if perm not in RWX 1 if perm is zero 2 if SDcur has insufficient perms
SCGrant	Cell description invalid	_INV_CELL_STATE	0 if cell is invalid
SCRecv	Illegal address	_ILL_ADDR	addr
SCRecv	Cell description invalid	_INV_CELL_STATE	0 if cell is invalid
SCRecv	Illegal permissions	_ILL_PERM	(type << 8) | perm type = 0 if perm not in RWX 1 if perm is zero 3 if requested perms not subset of granted perms
SCRecv	Invalid SD	_INV_SDID	(type << 32) | SD type = 0 if SD source too high, SD=SDsrc 1 if grant SD != SDcur, SD=SDcur
SCProtect	Illegal address	_ILL_ADDR	addr
SCProtect	Illegal permissions	_ILL_PERM	(type << 8) | perm type = 0 if perm not in RWX 2 if SDcur has insufficient perms
SCProtect	Cell description invalid	_INV_CELL_STATE	0 if cell is invalid
SCTfer	Illegal address	_ILL_ADDR	addr
SCTfer	Target too high	_INV_SDID	SDtgt
SCTfer	Illegal permissions	_ILL_PERM	(type << 8) | perm type = 0 if perm not in RWX 1 if perm is zero 2 if SDcur has insufficient perms
SCTfer	Cell description invalid	_INV_CELL_STATE	0 if cell is invalid
SCInval	Illegal address	_ILL_ADDR	addr
SCInval	Cell description invalid	_INV_CELL_STATE	0 if cell is already invalid 2 if other SD has non-zero perms 3 outstanding grants for cell
SCReval	Illegal address	_ILL_ADDR	addr
SCReval	Cell description invalid	_INV_CELL_STATE	1 if cell is already valid
SCReval	Illegal permissions	_ILL_PERM	(type << 8) | perm type = 0 if perm not in RWX 1 if perm is zero
SDSwitch	Illegal address	_ILL_ADDR	addr
SDSwitch	Illegal permissions	_ILL_PERM	(type << 8) | perm type= 4 for illegal sdswitch
SDSwitch	Target too high	_INV_SDID	SDtgt
SDSwitch	Illegal target instruction	_ILL_TGT	Target addr

Numeric values for scause shown below.

scause cause	scause Exception Code
_ILL_ADDR	0x18
_ILL_PERM	0x19
_INV_SDID	0x1a
_INV_CELL_STATE	0x1b
_ILL_TGT	0x1c