Root/Documentation/frv/kernel-ABI.txt

1            =================================
2            INTERNAL KERNEL ABI FOR FR-V ARCH
3            =================================
4
5The internal FRV kernel ABI is not quite the same as the userspace ABI. A
6number of the registers are used for special purposed, and the ABI is not
7consistent between modules vs core, and MMU vs no-MMU.
8
9This partly stems from the fact that FRV CPUs do not have a separate
10supervisor stack pointer, and most of them do not have any scratch
11registers, thus requiring at least one general purpose register to be
12clobbered in such an event. Also, within the kernel core, it is possible to
13simply jump or call directly between functions using a relative offset.
14This cannot be extended to modules for the displacement is likely to be too
15far. Thus in modules the address of a function to call must be calculated
16in a register and then used, requiring two extra instructions.
17
18This document has the following sections:
19
20 (*) System call register ABI
21 (*) CPU operating modes
22 (*) Internal kernel-mode register ABI
23 (*) Internal debug-mode register ABI
24 (*) Virtual interrupt handling
25
26
27========================
28SYSTEM CALL REGISTER ABI
29========================
30
31When a system call is made, the following registers are effective:
32
33    REGISTERS CALL RETURN
34    =============== ======================= =======================
35    GR7 System call number Preserved
36    GR8 Syscall arg #1 Return value
37    GR9-GR13 Syscall arg #2-6 Preserved
38
39
40===================
41CPU OPERATING MODES
42===================
43
44The FR-V CPU has three basic operating modes. In order of increasing
45capability:
46
47  (1) User mode.
48
49      Basic userspace running mode.
50
51  (2) Kernel mode.
52
53      Normal kernel mode. There are many additional control registers
54      available that may be accessed in this mode, in addition to all the
55      stuff available to user mode. This has two submodes:
56
57      (a) Exceptions enabled (PSR.T == 1).
58
59      Exceptions will invoke the appropriate normal kernel mode
60      handler. On entry to the handler, the PSR.T bit will be cleared.
61
62      (b) Exceptions disabled (PSR.T == 0).
63
64      No exceptions or interrupts may happen. Any mandatory exceptions
65      will cause the CPU to halt unless the CPU is told to jump into
66      debug mode instead.
67
68  (3) Debug mode.
69
70      No exceptions may happen in this mode. Memory protection and
71      management exceptions will be flagged for later consideration, but
72      the exception handler won't be invoked. Debugging traps such as
73      hardware breakpoints and watchpoints will be ignored. This mode is
74      entered only by debugging events obtained from the other two modes.
75
76      All kernel mode registers may be accessed, plus a few extra debugging
77      specific registers.
78
79
80=================================
81INTERNAL KERNEL-MODE REGISTER ABI
82=================================
83
84There are a number of permanent register assignments that are set up by
85entry.S in the exception prologue. Note that there is a complete set of
86exception prologues for each of user->kernel transition and kernel->kernel
87transition. There are also user->debug and kernel->debug mode transition
88prologues.
89
90
91    REGISTER FLAVOUR USE
92    =============== ======= ==============================================
93    GR1 Supervisor stack pointer
94    GR15 Current thread info pointer
95    GR16 GP-Rel base register for small data
96    GR28 Current exception frame pointer (__frame)
97    GR29 Current task pointer (current)
98    GR30 Destroyed by kernel mode entry
99    GR31 NOMMU Destroyed by debug mode entry
100    GR31 MMU Destroyed by TLB miss kernel mode entry
101    CCR.ICC2 Virtual interrupt disablement tracking
102    CCCR.CC3 Cleared by exception prologue
103                (atomic op emulation)
104    SCR0 MMU See mmu-layout.txt.
105    SCR1 MMU See mmu-layout.txt.
106    SCR2 MMU Save for EAR0 (destroyed by icache insns
107                           in debug mode)
108    SCR3 MMU Save for GR31 during debug exceptions
109    DAMR/IAMR NOMMU Fixed memory protection layout.
110    DAMR/IAMR MMU See mmu-layout.txt.
111
112
113Certain registers are also used or modified across function calls:
114
115    REGISTER CALL RETURN
116    =============== =============================== ======================
117    GR0 Fixed Zero -
118    GR2 Function call frame pointer
119    GR3 Special Preserved
120    GR3-GR7 - Clobbered
121    GR8 Function call arg #1 Return value
122                            (or clobbered)
123    GR9 Function call arg #2 Return value MSW
124                            (or clobbered)
125    GR10-GR13 Function call arg #3-#6 Clobbered
126    GR14 - Clobbered
127    GR15-GR16 Special Preserved
128    GR17-GR27 - Preserved
129    GR28-GR31 Special Only accessed
130                            explicitly
131    LR Return address after CALL Clobbered
132    CCR/CCCR - Mostly Clobbered
133
134
135================================
136INTERNAL DEBUG-MODE REGISTER ABI
137================================
138
139This is the same as the kernel-mode register ABI for functions calls. The
140difference is that in debug-mode there's a different stack and a different
141exception frame. Almost all the global registers from kernel-mode
142(including the stack pointer) may be changed.
143
144    REGISTER FLAVOUR USE
145    =============== ======= ==============================================
146    GR1 Debug stack pointer
147    GR16 GP-Rel base register for small data
148    GR31 Current debug exception frame pointer
149                (__debug_frame)
150    SCR3 MMU Saved value of GR31
151
152
153Note that debug mode is able to interfere with the kernel's emulated atomic
154ops, so it must be exceedingly careful not to do any that would interact
155with the main kernel in this regard. Hence the debug mode code (gdbstub) is
156almost completely self-contained. The only external code used is the
157sprintf family of functions.
158
159Furthermore, break.S is so complicated because single-step mode does not
160switch off on entry to an exception. That means unless manually disabled,
161single-stepping will blithely go on stepping into things like interrupts.
162See gdbstub.txt for more information.
163
164
165==========================
166VIRTUAL INTERRUPT HANDLING
167==========================
168
169Because accesses to the PSR is so slow, and to disable interrupts we have
170to access it twice (once to read and once to write), we don't actually
171disable interrupts at all if we don't have to. What we do instead is use
172the ICC2 condition code flags to note virtual disablement, such that if we
173then do take an interrupt, we note the flag, really disable interrupts, set
174another flag and resume execution at the point the interrupt happened.
175Setting condition flags as a side effect of an arithmetic or logical
176instruction is really fast. This use of the ICC2 only occurs within the
177kernel - it does not affect userspace.
178
179The flags we use are:
180
181 (*) CCR.ICC2.Z [Zero flag]
182
183     Set to virtually disable interrupts, clear when interrupts are
184     virtually enabled. Can be modified by logical instructions without
185     affecting the Carry flag.
186
187 (*) CCR.ICC2.C [Carry flag]
188
189     Clear to indicate hardware interrupts are really disabled, set otherwise.
190
191
192What happens is this:
193
194 (1) Normal kernel-mode operation.
195
196    ICC2.Z is 0, ICC2.C is 1.
197
198 (2) An interrupt occurs. The exception prologue examines ICC2.Z and
199     determines that nothing needs doing. This is done simply with an
200     unlikely BEQ instruction.
201
202 (3) The interrupts are disabled (local_irq_disable)
203
204    ICC2.Z is set to 1.
205
206 (4) If interrupts were then re-enabled (local_irq_enable):
207
208    ICC2.Z would be set to 0.
209
210     A TIHI #2 instruction (trap #2 if condition HI - Z==0 && C==0) would
211     be used to trap if interrupts were now virtually enabled, but
212     physically disabled - which they're not, so the trap isn't taken. The
213     kernel would then be back to state (1).
214
215 (5) An interrupt occurs. The exception prologue examines ICC2.Z and
216     determines that the interrupt shouldn't actually have happened. It
217     jumps aside, and there disabled interrupts by setting PSR.PIL to 14
218     and then it clears ICC2.C.
219
220 (6) If interrupts were then saved and disabled again (local_irq_save):
221
222    ICC2.Z would be shifted into the save variable and masked off
223    (giving a 1).
224
225    ICC2.Z would then be set to 1 (thus unchanged), and ICC2.C would be
226    unaffected (ie: 0).
227
228 (7) If interrupts were then restored from state (6) (local_irq_restore):
229
230    ICC2.Z would be set to indicate the result of XOR'ing the saved
231    value (ie: 1) with 1, which gives a result of 0 - thus leaving
232    ICC2.Z set.
233
234    ICC2.C would remain unaffected (ie: 0).
235
236     A TIHI #2 instruction would be used to again assay the current state,
237     but this would do nothing as Z==1.
238
239 (8) If interrupts were then enabled (local_irq_enable):
240
241    ICC2.Z would be cleared. ICC2.C would be left unaffected. Both
242    flags would now be 0.
243
244     A TIHI #2 instruction again issued to assay the current state would
245     then trap as both Z==0 [interrupts virtually enabled] and C==0
246     [interrupts really disabled] would then be true.
247
248 (9) The trap #2 handler would simply enable hardware interrupts
249     (set PSR.PIL to 0), set ICC2.C to 1 and return.
250
251(10) Immediately upon returning, the pending interrupt would be taken.
252
253(11) The interrupt handler would take the path of actually processing the
254     interrupt (ICC2.Z is clear, BEQ fails as per step (2)).
255
256(12) The interrupt handler would then set ICC2.C to 1 since hardware
257     interrupts are definitely enabled - or else the kernel wouldn't be here.
258
259(13) On return from the interrupt handler, things would be back to state (1).
260
261This trap (#2) is only available in kernel mode. In user mode it will
262result in SIGILL.
263

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