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=================================
INTERNAL KERNEL ABI FOR FR-V ARCH
=================================
The internal FRV kernel ABI is not quite the same as the userspace ABI. A number of the registers
are used for special purposed, and the ABI is not consistent between modules vs core, and MMU vs
no-MMU.
This partly stems from the fact that FRV CPUs do not have a separate supervisor stack pointer, and
most of them do not have any scratch registers, thus requiring at least one general purpose
register to be clobbered in such an event. Also, within the kernel core, it is possible to simply
jump or call directly between functions using a relative offset. This cannot be extended to modules
for the displacement is likely to be too far. Thus in modules the address of a function to call
must be calculated in a register and then used, requiring two extra instructions.
This document has the following sections:
(*) System call register ABI
(*) CPU operating modes
(*) Internal kernel-mode register ABI
(*) Internal debug-mode register ABI
(*) Virtual interrupt handling
========================
SYSTEM CALL REGISTER ABI
========================
When a system call is made, the following registers are effective:
REGISTERS CALL RETURN
=============== ======================= =======================
GR7 System call number Preserved
GR8 Syscall arg #1 Return value
GR9-GR13 Syscall arg #2-6 Preserved
===================
CPU OPERATING MODES
===================
The FR-V CPU has three basic operating modes. In order of increasing capability:
(1) User mode.
Basic userspace running mode.
(2) Kernel mode.
Normal kernel mode. There are many additional control registers available that may be
accessed in this mode, in addition to all the stuff available to user mode. This has two
submodes:
(a) Exceptions enabled (PSR.T == 1).
Exceptions will invoke the appropriate normal kernel mode handler. On entry to the
handler, the PSR.T bit will be cleared.
(b) Exceptions disabled (PSR.T == 0).
No exceptions or interrupts may happen. Any mandatory exceptions will cause the CPU to
halt unless the CPU is told to jump into debug mode instead.
(3) Debug mode.
No exceptions may happen in this mode. Memory protection and management exceptions will be
flagged for later consideration, but the exception handler won't be invoked. Debugging traps
such as hardware breakpoints and watchpoints will be ignored. This mode is entered only by
debugging events obtained from the other two modes.
All kernel mode registers may be accessed, plus a few extra debugging specific registers.
=================================
INTERNAL KERNEL-MODE REGISTER ABI
=================================
There are a number of permanent register assignments that are set up by entry.S in the exception
prologue. Note that there is a complete set of exception prologues for each of user->kernel
transition and kernel->kernel transition. There are also user->debug and kernel->debug mode
transition prologues.
REGISTER FLAVOUR USE
=============== ======= ====================================================
GR1 Supervisor stack pointer
GR15 Current thread info pointer
GR16 GP-Rel base register for small data
GR28 Current exception frame pointer (__frame)
GR29 Current task pointer (current)
GR30 Destroyed by kernel mode entry
GR31 NOMMU Destroyed by debug mode entry
GR31 MMU Destroyed by TLB miss kernel mode entry
CCR.ICC2 Virtual interrupt disablement tracking
CCCR.CC3 Cleared by exception prologue (atomic op emulation)
SCR0 MMU See mmu-layout.txt.
SCR1 MMU See mmu-layout.txt.
SCR2 MMU Save for EAR0 (destroyed by icache insns in debug mode)
SCR3 MMU Save for GR31 during debug exceptions
DAMR/IAMR NOMMU Fixed memory protection layout.
DAMR/IAMR MMU See mmu-layout.txt.
Certain registers are also used or modified across function calls:
REGISTER CALL RETURN
=============== =============================== ===============================
GR0 Fixed Zero -
GR2 Function call frame pointer
GR3 Special Preserved
GR3-GR7 - Clobbered
GR8 Function call arg #1 Return value (or clobbered)
GR9 Function call arg #2 Return value MSW (or clobbered)
GR10-GR13 Function call arg #3-#6 Clobbered
GR14 - Clobbered
GR15-GR16 Special Preserved
GR17-GR27 - Preserved
GR28-GR31 Special Only accessed explicitly
LR Return address after CALL Clobbered
CCR/CCCR - Mostly Clobbered
================================
INTERNAL DEBUG-MODE REGISTER ABI
================================
This is the same as the kernel-mode register ABI for functions calls. The difference is that in
debug-mode there's a different stack and a different exception frame. Almost all the global
registers from kernel-mode (including the stack pointer) may be changed.
REGISTER FLAVOUR USE
=============== ======= ====================================================
GR1 Debug stack pointer
GR16 GP-Rel base register for small data
GR31 Current debug exception frame pointer (__debug_frame)
SCR3 MMU Saved value of GR31
Note that debug mode is able to interfere with the kernel's emulated atomic ops, so it must be
exceedingly careful not to do any that would interact with the main kernel in this regard. Hence
the debug mode code (gdbstub) is almost completely self-contained. The only external code used is
the sprintf family of functions.
Futhermore, break.S is so complicated because single-step mode does not switch off on entry to an
exception. That means unless manually disabled, single-stepping will blithely go on stepping into
things like interrupts. See gdbstub.txt for more information.
==========================
VIRTUAL INTERRUPT HANDLING
==========================
Because accesses to the PSR is so slow, and to disable interrupts we have to access it twice (once
to read and once to write), we don't actually disable interrupts at all if we don't have to. What
we do instead is use the ICC2 condition code flags to note virtual disablement, such that if we
then do take an interrupt, we note the flag, really disable interrupts, set another flag and resume
execution at the point the interrupt happened. Setting condition flags as a side effect of an
arithmetic or logical instruction is really fast. This use of the ICC2 only occurs within the
kernel - it does not affect userspace.
The flags we use are:
(*) CCR.ICC2.Z [Zero flag]
Set to virtually disable interrupts, clear when interrupts are virtually enabled. Can be
modified by logical instructions without affecting the Carry flag.
(*) CCR.ICC2.C [Carry flag]
Clear to indicate hardware interrupts are really disabled, set otherwise.
What happens is this:
(1) Normal kernel-mode operation.
ICC2.Z is 0, ICC2.C is 1.
(2) An interrupt occurs. The exception prologue examines ICC2.Z and determines that nothing needs
doing. This is done simply with an unlikely BEQ instruction.
(3) The interrupts are disabled (local_irq_disable)
ICC2.Z is set to 1.
(4) If interrupts were then re-enabled (local_irq_enable):
ICC2.Z would be set to 0.
A TIHI #2 instruction (trap #2 if condition HI - Z==0 && C==0) would be used to trap if
interrupts were now virtually enabled, but physically disabled - which they're not, so the
trap isn't taken. The kernel would then be back to state (1).
(5) An interrupt occurs. The exception prologue examines ICC2.Z and determines that the interrupt
shouldn't actually have happened. It jumps aside, and there disabled interrupts by setting
PSR.PIL to 14 and then it clears ICC2.C.
(6) If interrupts were then saved and disabled again (local_irq_save):
ICC2.Z would be shifted into the save variable and masked off (giving a 1).
ICC2.Z would then be set to 1 (thus unchanged), and ICC2.C would be unaffected (ie: 0).
(7) If interrupts were then restored from state (6) (local_irq_restore):
ICC2.Z would be set to indicate the result of XOR'ing the saved value (ie: 1) with 1, which
gives a result of 0 - thus leaving ICC2.Z set.
ICC2.C would remain unaffected (ie: 0).
A TIHI #2 instruction would be used to again assay the current state, but this would do
nothing as Z==1.
(8) If interrupts were then enabled (local_irq_enable):
ICC2.Z would be cleared. ICC2.C would be left unaffected. Both flags would now be 0.
A TIHI #2 instruction again issued to assay the current state would then trap as both Z==0
[interrupts virtually enabled] and C==0 [interrupts really disabled] would then be true.
(9) The trap #2 handler would simply enable hardware interrupts (set PSR.PIL to 0), set ICC2.C to
1 and return.
(10) Immediately upon returning, the pending interrupt would be taken.
(11) The interrupt handler would take the path of actually processing the interrupt (ICC2.Z is
clear, BEQ fails as per step (2)).
(12) The interrupt handler would then set ICC2.C to 1 since hardware interrupts are definitely
enabled - or else the kernel wouldn't be here.
(13) On return from the interrupt handler, things would be back to state (1).
This trap (#2) is only available in kernel mode. In user mode it will result in SIGILL.
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