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+ ==========================
+ KERNEL ABIS FOR METAG ARCH
+ ==========================
+
+This document describes the Linux ABIs for the metag architecture, and has the
+following sections:
+
+ (*) Outline of registers
+ (*) Userland registers
+ (*) Kernel registers
+ (*) System call ABI
+ (*) Calling conventions
+
+
+====================
+OUTLINE OF REGISTERS
+====================
+
+The main Meta core registers are arranged in units:
+
+ UNIT Type DESCRIPTION GP EXT PRIV GLOBAL
+ ======= ======= =============== ======= ======= ======= =======
+ CT Special Control unit
+ D0 General Data unit 0 0-7 8-15 16-31 16-31
+ D1 General Data unit 1 0-7 8-15 16-31 16-31
+ A0 General Address unit 0 0-3 4-7 8-15 8-15
+ A1 General Address unit 1 0-3 4-7 8-15 8-15
+ PC Special PC unit 0 1
+ PORT Special Ports
+ TR Special Trigger unit 0-7
+ TT Special Trace unit 0-5
+ FX General FP unit 0-15
+
+GP registers form part of the main context.
+
+Extended context registers (EXT) may not be present on all hardware threads and
+can be context switched if support is enabled and the appropriate bits are set
+in e.g. the D0.8 register to indicate what extended state to preserve.
+
+Global registers are shared between threads and are privilege protected.
+
+See arch/metag/include/asm/metag_regs.h for definitions relating to core
+registers and the fields and bits they contain. See the TRMs for further details
+about special registers.
+
+Several special registers are preserved in the main context, these are the
+interesting ones:
+
+ REG (ALIAS) PURPOSE
+ ======================= ===============================================
+ CT.1 (TXMODE) Processor mode bits (particularly for DSP)
+ CT.2 (TXSTATUS) Condition flags and LSM_STEP (MGET/MSET step)
+ CT.3 (TXRPT) Branch repeat counter
+ PC.0 (PC) Program counter
+
+Some of the general registers have special purposes in the ABI and therefore
+have aliases:
+
+ D0 REG (ALIAS) PURPOSE D1 REG (ALIAS) PURPOSE
+ =============== =============== =============== =======================
+ D0.0 (D0Re0) 32bit result D1.0 (D1Re0) Top half of 64bit result
+ D0.1 (D0Ar6) Argument 6 D1.1 (D1Ar5) Argument 5
+ D0.2 (D0Ar4) Argument 4 D1.2 (D1Ar3) Argument 3
+ D0.3 (D0Ar2) Argument 2 D1.3 (D1Ar1) Argument 1
+ D0.4 (D0FrT) Frame temp D1.4 (D1RtP) Return pointer
+ D0.5 Call preserved D1.5 Call preserved
+ D0.6 Call preserved D1.6 Call preserved
+ D0.7 Call preserved D1.7 Call preserved
+
+ A0 REG (ALIAS) PURPOSE A1 REG (ALIAS) PURPOSE
+ =============== =============== =============== =======================
+ A0.0 (A0StP) Stack pointer A1.0 (A1GbP) Global base pointer
+ A0.1 (A0FrP) Frame pointer A1.1 (A1LbP) Local base pointer
+ A0.2 A1.2
+ A0.3 A1.3
+
+
+==================
+USERLAND REGISTERS
+==================
+
+All the general purpose D0, D1, A0, A1 registers are preserved when entering the
+kernel (including asynchronous events such as interrupts and timer ticks) except
+the following which have special purposes in the ABI:
+
+ REGISTERS WHEN STATUS PURPOSE
+ =============== ======= =============== ===============================
+ D0.8 DSP Preserved ECH, determines what extended
+ DSP state to preserve.
+ A0.0 (A0StP) ALWAYS Preserved Stack >= A0StP may be clobbered
+ at any time by the creation of a
+ signal frame.
+ A1.0 (A1GbP) SMP Clobbered Used as temporary for loading
+ kernel stack pointer and saving
+ core context.
+ A0.15 !SMP Protected Stores kernel stack pointer.
+ A1.15 ALWAYS Protected Stores kernel base pointer.
+
+On UP A0.15 is used to store the kernel stack pointer for storing the userland
+context. A0.15 is global between hardware threads though which means it cannot
+be used on SMP for this purpose. Since no protected local registers are
+available A1GbP is reserved for use as a temporary to allow a percpu stack
+pointer to be loaded for storing the rest of the context.
+
+
+================
+KERNEL REGISTERS
+================
+
+When in the kernel the following registers have special purposes in the ABI:
+
+ REGISTERS WHEN STATUS PURPOSE
+ =============== ======= =============== ===============================
+ A0.0 (A0StP) ALWAYS Preserved Stack >= A0StP may be clobbered
+ at any time by the creation of
+ an irq signal frame.
+ A1.0 (A1GbP) ALWAYS Preserved Reserved (kernel base pointer).
+
+
+===============
+SYSTEM CALL ABI
+===============
+
+When a system call is made, the following registers are effective:
+
+ REGISTERS CALL RETURN
+ =============== ======================= ===============================
+ D0.0 (D0Re0) Return value (or -errno)
+ D1.0 (D1Re0) System call number Clobbered
+ D0.1 (D0Ar6) Syscall arg #6 Preserved
+ D1.1 (D1Ar5) Syscall arg #5 Preserved
+ D0.2 (D0Ar4) Syscall arg #4 Preserved
+ D1.2 (D1Ar3) Syscall arg #3 Preserved
+ D0.3 (D0Ar2) Syscall arg #2 Preserved
+ D1.3 (D1Ar1) Syscall arg #1 Preserved
+
+Due to the limited number of argument registers and some system calls with badly
+aligned 64-bit arguments, 64-bit values are always packed in consecutive
+arguments, even if this is contrary to the normal calling conventions (where the
+two halves would go in a matching pair of data registers).
+
+For example fadvise64_64 usually has the signature:
+
+ long sys_fadvise64_64(i32 fd, i64 offs, i64 len, i32 advice);
+
+But for metag fadvise64_64 is wrapped so that the 64-bit arguments are packed:
+
+ long sys_fadvise64_64_metag(i32 fd, i32 offs_lo,
+ i32 offs_hi, i32 len_lo,
+ i32 len_hi, i32 advice)
+
+So the arguments are packed in the registers like this:
+
+ D0 REG (ALIAS) VALUE D1 REG (ALIAS) VALUE
+ =============== =============== =============== =======================
+ D0.1 (D0Ar6) advice D1.1 (D1Ar5) hi(len)
+ D0.2 (D0Ar4) lo(len) D1.2 (D1Ar3) hi(offs)
+ D0.3 (D0Ar2) lo(offs) D1.3 (D1Ar1) fd
+
+
+===================
+CALLING CONVENTIONS
+===================
+
+These calling conventions apply to both user and kernel code. The stack grows
+from low addresses to high addresses in the metag ABI. The stack pointer (A0StP)
+should always point to the next free address on the stack and should at all
+times be 64-bit aligned. The following registers are effective at the point of a
+call:
+
+ REGISTERS CALL RETURN
+ =============== ======================= ===============================
+ D0.0 (D0Re0) 32bit return value
+ D1.0 (D1Re0) Upper half of 64bit return value
+ D0.1 (D0Ar6) 32bit argument #6 Clobbered
+ D1.1 (D1Ar5) 32bit argument #5 Clobbered
+ D0.2 (D0Ar4) 32bit argument #4 Clobbered
+ D1.2 (D1Ar3) 32bit argument #3 Clobbered
+ D0.3 (D0Ar2) 32bit argument #2 Clobbered
+ D1.3 (D1Ar1) 32bit argument #1 Clobbered
+ D0.4 (D0FrT) Clobbered
+ D1.4 (D1RtP) Return pointer Clobbered
+ D{0-1}.{5-7} Preserved
+ A0.0 (A0StP) Stack pointer Preserved
+ A1.0 (A0GbP) Preserved
+ A0.1 (A0FrP) Frame pointer Preserved
+ A1.1 (A0LbP) Preserved
+ A{0-1},{2-3} Clobbered
+
+64-bit arguments are placed in matching pairs of registers (i.e. the same
+register number in both D0 and D1 units), with the least significant half in D0
+and the most significant half in D1, leaving a gap where necessary. Futher
+arguments are stored on the stack in reverse order (earlier arguments at higher
+addresses):
+
+ ADDRESS 0 1 2 3 4 5 6 7
+ =============== ===== ===== ===== ===== ===== ===== ===== =====
+ A0StP -->
+ A0StP-0x08 32bit argument #8 32bit argument #7
+ A0StP-0x10 32bit argument #10 32bit argument #9
+
+Function prologues tend to look a bit like this:
+
+ /* If frame pointer in use, move it to frame temp register so it can be
+ easily pushed onto stack */
+ MOV D0FrT,A0FrP
+
+ /* If frame pointer in use, set it to stack pointer */
+ ADD A0FrP,A0StP,#0
+
+ /* Preserve D0FrT, D1RtP, D{0-1}.{5-7} on stack, incrementing A0StP */
+ MSETL [A0StP++],D0FrT,D0.5,D0.6,D0.7
+
+ /* Allocate some stack space for local variables */
+ ADD A0StP,A0StP,#0x10
+
+At this point the stack would look like this:
+
+ ADDRESS 0 1 2 3 4 5 6 7
+ =============== ===== ===== ===== ===== ===== ===== ===== =====
+ A0StP -->
+ A0StP-0x08
+ A0StP-0x10
+ A0StP-0x18 Old D0.7 Old D1.7
+ A0StP-0x20 Old D0.6 Old D1.6
+ A0StP-0x28 Old D0.5 Old D1.5
+ A0FrP --> Old A0FrP (frame ptr) Old D1RtP (return ptr)
+ A0FrP-0x08 32bit argument #8 32bit argument #7
+ A0FrP-0x10 32bit argument #10 32bit argument #9
+
+Function epilogues tend to differ depending on the use of a frame pointer. An
+example of a frame pointer epilogue:
+
+ /* Restore D0FrT, D1RtP, D{0-1}.{5-7} from stack, incrementing A0FrP */
+ MGETL D0FrT,D0.5,D0.6,D0.7,[A0FrP++]
+ /* Restore stack pointer to where frame pointer was before increment */
+ SUB A0StP,A0FrP,#0x20
+ /* Restore frame pointer from frame temp */
+ MOV A0FrP,D0FrT
+ /* Return to caller via restored return pointer */
+ MOV PC,D1RtP
+
+If the function hasn't touched the frame pointer, MGETL cannot be safely used
+with A0StP as it always increments and that would expose the stack to clobbering
+by interrupts (kernel) or signals (user). Therefore it's common to see the MGETL
+split into separate GETL instructions:
+
+ /* Restore D0FrT, D1RtP, D{0-1}.{5-7} from stack */
+ GETL D0FrT,D1RtP,[A0StP+#-0x30]
+ GETL D0.5,D1.5,[A0StP+#-0x28]
+ GETL D0.6,D1.6,[A0StP+#-0x20]
+ GETL D0.7,D1.7,[A0StP+#-0x18]
+ /* Restore stack pointer */
+ SUB A0StP,A0StP,#0x30
+ /* Return to caller via restored return pointer */
+ MOV PC,D1RtP