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diff --git a/Documentation/metag/kernel-ABI.txt b/Documentation/metag/kernel-ABI.txt new file mode 100644 index 000000000000..7b8dee83b9c1 --- /dev/null +++ b/Documentation/metag/kernel-ABI.txt @@ -0,0 +1,256 @@ + ========================== + 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 |