ARC: MMU Exception Handling

* MMU I-TLB / D-TLB Miss Exceptions
  - Fast Path TLB Refill Handler
  - slowpath TLB creation via do_page_fault() -> update_mmu_cache()
* Duplicate PD Exception Handler

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

2 files changed, 618 insertions, 0 deletions

diff --git a/arch/arc/mm/tlb.c b/arch/arc/mm/tlb.c
index f1edae2410a7..404e5be4f704 100644
--- a/arch/arc/mm/tlb.c
+++ b/arch/arc/mm/tlb.c
@@ -21,3 +21,270 @@ int asid_cache = FIRST_ASID;
  * see get_new_mmu_context (asm-arc/mmu_context.h)
  */
 struct mm_struct *asid_mm_map[NUM_ASID + 1];
+
+
+/*
+ * Routine to create a TLB entry
+ */
+void create_tlb(struct vm_area_struct *vma, unsigned long address, pte_t *ptep)
+{
+        unsigned long flags;
+        unsigned int idx, asid_or_sasid;
+        unsigned long pd0_flags;
+
+        /*
+         * create_tlb() assumes that current->mm == vma->mm, since
+         * -it ASID for TLB entry is fetched from MMU ASID reg (valid for curr)
+         * -completes the lazy write to SASID reg (again valid for curr tsk)
+         *
+         * Removing the assumption involves
+         * -Using vma->mm->context{ASID,SASID}, as opposed to MMU reg.
+         * -Fix the TLB paranoid debug code to not trigger false negatives.
+         * -More importantly it makes this handler inconsistent with fast-path
+         *  TLB Refill handler which always deals with "current"
+         *
+         * Lets see the use cases when current->mm != vma->mm and we land here
+         *  1. execve->copy_strings()->__get_user_pages->handle_mm_fault
+         *     Here VM wants to pre-install a TLB entry for user stack while
+         *     current->mm still points to pre-execve mm (hence the condition).
+         *     However the stack vaddr is soon relocated (randomization) and
+         *     move_page_tables() tries to undo that TLB entry.
+         *     Thus not creating TLB entry is not any worse.
+         *
+         *  2. ptrace(POKETEXT) causes a CoW - debugger(current) inserting a
+         *     breakpoint in debugged task. Not creating a TLB now is not
+         *     performance critical.
+         *
+         * Both the cases above are not good enough for code churn.
+         */
+        if (current->active_mm != vma->vm_mm)
+                return;
+
+        local_irq_save(flags);
+
+        tlb_paranoid_check(vma->vm_mm->context.asid, address);
+
+        address &= PAGE_MASK;
+
+        /* update this PTE credentials */
+        pte_val(*ptep) |= (_PAGE_PRESENT | _PAGE_ACCESSED);
+
+        /* Create HW TLB entry Flags (in PD0) from PTE Flags */
+#if (CONFIG_ARC_MMU_VER <= 2)
+        pd0_flags = ((pte_val(*ptep) & PTE_BITS_IN_PD0) >> 1);
+#else
+        pd0_flags = ((pte_val(*ptep) & PTE_BITS_IN_PD0));
+#endif
+
+        /* ASID for this task */
+        asid_or_sasid = read_aux_reg(ARC_REG_PID) & 0xff;
+
+        write_aux_reg(ARC_REG_TLBPD0, address | pd0_flags | asid_or_sasid);
+
+        /* Load remaining info in PD1 (Page Frame Addr and Kx/Kw/Kr Flags) */
+        write_aux_reg(ARC_REG_TLBPD1, (pte_val(*ptep) & PTE_BITS_IN_PD1));
+
+        /* First verify if entry for this vaddr+ASID already exists */
+        write_aux_reg(ARC_REG_TLBCOMMAND, TLBProbe);
+        idx = read_aux_reg(ARC_REG_TLBINDEX);
+
+        /*
+         * If Not already present get a free slot from MMU.
+         * Otherwise, Probe would have located the entry and set INDEX Reg
+         * with existing location. This will cause Write CMD to over-write
+         * existing entry with new PD0 and PD1
+         */
+        if (likely(idx & TLB_LKUP_ERR))
+                write_aux_reg(ARC_REG_TLBCOMMAND, TLBGetIndex);
+
+        /*
+         * Commit the Entry to MMU
+         * It doesnt sound safe to use the TLBWriteNI cmd here
+         * which doesn't flush uTLBs. I'd rather be safe than sorry.
+         */
+        write_aux_reg(ARC_REG_TLBCOMMAND, TLBWrite);
+
+        local_irq_restore(flags);
+}
+
+/* arch hook called by core VM at the end of handle_mm_fault( ),
+ * when a new PTE is entered in Page Tables or an existing one
+ * is modified. We aggresively pre-install a TLB entry
+ */
+
+void update_mmu_cache(struct vm_area_struct *vma, unsigned long vaddress,
+                      pte_t *ptep)
+{
+
+        create_tlb(vma, vaddress, ptep);
+}
+
+/* Read the Cache Build Confuration Registers, Decode them and save into
+ * the cpuinfo structure for later use.
+ * No Validation is done here, simply read/convert the BCRs
+ */
+void __init read_decode_mmu_bcr(void)
+{
+        unsigned int tmp;
+        struct bcr_mmu_1_2 *mmu2;       /* encoded MMU2 attr */
+        struct bcr_mmu_3 *mmu3;         /* encoded MMU3 attr */
+        struct cpuinfo_arc_mmu *mmu = &cpuinfo_arc700[smp_processor_id()].mmu;
+
+        tmp = read_aux_reg(ARC_REG_MMU_BCR);
+        mmu->ver = (tmp >> 24);
+
+        if (mmu->ver <= 2) {
+                mmu2 = (struct bcr_mmu_1_2 *)&tmp;
+                mmu->pg_sz = PAGE_SIZE;
+                mmu->sets = 1 << mmu2->sets;
+                mmu->ways = 1 << mmu2->ways;
+                mmu->u_dtlb = mmu2->u_dtlb;
+                mmu->u_itlb = mmu2->u_itlb;
+        } else {
+                mmu3 = (struct bcr_mmu_3 *)&tmp;
+                mmu->pg_sz = 512 << mmu3->pg_sz;
+                mmu->sets = 1 << mmu3->sets;
+                mmu->ways = 1 << mmu3->ways;
+                mmu->u_dtlb = mmu3->u_dtlb;
+                mmu->u_itlb = mmu3->u_itlb;
+        }
+
+        mmu->num_tlb = mmu->sets * mmu->ways;
+}
+
+void __init arc_mmu_init(void)
+{
+        /*
+         * ASID mgmt data structures are compile time init
+         *  asid_cache = FIRST_ASID and asid_mm_map[] all zeroes
+         */
+
+        local_flush_tlb_all();
+
+        /* Enable the MMU */
+        write_aux_reg(ARC_REG_PID, MMU_ENABLE);
+}
+
+/*
+ * TLB Programmer's Model uses Linear Indexes: 0 to {255, 511} for 128 x {2,4}
+ * The mapping is Column-first.
+ *              ---------------------   -----------
+ *              |way0|way1|way2|way3|   |way0|way1|
+ *              ---------------------   -----------
+ * [set0]       |  0 |  1 |  2 |  3 |   |  0 |  1 |
+ * [set1]       |  4 |  5 |  6 |  7 |   |  2 |  3 |
+ *              ~                   ~   ~         ~
+ * [set127]     | 508| 509| 510| 511|   | 254| 255|
+ *              ---------------------   -----------
+ * For normal operations we don't(must not) care how above works since
+ * MMU cmd getIndex(vaddr) abstracts that out.
+ * However for walking WAYS of a SET, we need to know this
+ */
+#define SET_WAY_TO_IDX(mmu, set, way)  ((set) * mmu->ways + (way))
+
+/* Handling of Duplicate PD (TLB entry) in MMU.
+ * -Could be due to buggy customer tapeouts or obscure kernel bugs
+ * -MMU complaints not at the time of duplicate PD installation, but at the
+ *      time of lookup matching multiple ways.
+ * -Ideally these should never happen - but if they do - workaround by deleting
+ *      the duplicate one.
+ * -Knob to be verbose abt it.(TODO: hook them up to debugfs)
+ */
+volatile int dup_pd_verbose = 1;/* Be slient abt it or complain (default) */
+
+void do_tlb_overlap_fault(unsigned long cause, unsigned long address,
+                          struct pt_regs *regs)
+{
+        int set, way, n;
+        unsigned int pd0[4], pd1[4];    /* assume max 4 ways */
+        unsigned long flags, is_valid;
+        struct cpuinfo_arc_mmu *mmu = &cpuinfo_arc700[smp_processor_id()].mmu;
+
+        local_irq_save(flags);
+
+        /* re-enable the MMU */
+        write_aux_reg(ARC_REG_PID, MMU_ENABLE | read_aux_reg(ARC_REG_PID));
+
+        /* loop thru all sets of TLB */
+        for (set = 0; set < mmu->sets; set++) {
+
+                /* read out all the ways of current set */
+                for (way = 0, is_valid = 0; way < mmu->ways; way++) {
+                        write_aux_reg(ARC_REG_TLBINDEX,
+                                          SET_WAY_TO_IDX(mmu, set, way));
+                        write_aux_reg(ARC_REG_TLBCOMMAND, TLBRead);
+                        pd0[way] = read_aux_reg(ARC_REG_TLBPD0);
+                        pd1[way] = read_aux_reg(ARC_REG_TLBPD1);
+                        is_valid |= pd0[way] & _PAGE_PRESENT;
+                }
+
+                /* If all the WAYS in SET are empty, skip to next SET */
+                if (!is_valid)
+                        continue;
+
+                /* Scan the set for duplicate ways: needs a nested loop */
+                for (way = 0; way < mmu->ways; way++) {
+                        if (!pd0[way])
+                                continue;
+
+                        for (n = way + 1; n < mmu->ways; n++) {
+                                if ((pd0[way] & PAGE_MASK) ==
+                                    (pd0[n] & PAGE_MASK)) {
+
+                                        if (dup_pd_verbose) {
+                                                pr_info("Duplicate PD's @"
+                                                        "[%d:%d]/[%d:%d]\n",
+                                                     set, way, set, n);
+                                                pr_info("TLBPD0[%u]: %08x\n",
+                                                     way, pd0[way]);
+                                        }
+
+                                        /*
+                                         * clear entry @way and not @n. This is
+                                         * critical to our optimised loop
+                                         */
+                                        pd0[way] = pd1[way] = 0;
+                                        write_aux_reg(ARC_REG_TLBINDEX,
+                                                SET_WAY_TO_IDX(mmu, set, way));
+                                        __tlb_entry_erase();
+                                }
+                        }
+                }
+        }
+
+        local_irq_restore(flags);
+}
+
+/***********************************************************************
+ * Diagnostic Routines
+ *  -Called from Low Level TLB Hanlders if things don;t look good
+ **********************************************************************/
+
+#ifdef CONFIG_ARC_DBG_TLB_PARANOIA
+
+/*
+ * Low Level ASM TLB handler calls this if it finds that HW and SW ASIDS
+ * don't match
+ */
+void print_asid_mismatch(int is_fast_path)
+{
+        int pid_sw, pid_hw;
+        pid_sw = current->active_mm->context.asid;
+        pid_hw = read_aux_reg(ARC_REG_PID) & 0xff;
+
+        pr_emerg("ASID Mismatch in %s Path Handler: sw-pid=0x%x hw-pid=0x%x\n",
+               is_fast_path ? "Fast" : "Slow", pid_sw, pid_hw);
+
+        __asm__ __volatile__("flag 1");
+}
+
+void tlb_paranoid_check(unsigned int pid_sw, unsigned long addr)
+{
+        unsigned int pid_hw;
+
+        pid_hw = read_aux_reg(ARC_REG_PID) & 0xff;
+
+        if (addr < 0x70000000 && ((pid_hw != pid_sw) || (pid_sw == NO_ASID)))
+                print_asid_mismatch(0);
+}
+#endif
diff --git a/arch/arc/mm/tlbex.S b/arch/arc/mm/tlbex.S
new file mode 100644
index 000000000000..fc5b97129e55
--- /dev/null
+++ b/arch/arc/mm/tlbex.S
@@ -0,0 +1,351 @@
+/*
+ * TLB Exception Handling for ARC
+ *
+ * Copyright (C) 2004, 2007-2010, 2011-2012 Synopsys, Inc. (www.synopsys.com)
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * Vineetg: April 2011 :
+ *  -MMU v1: moved out legacy code into a seperate file
+ *  -MMU v3: PD{0,1} bits layout changed: They don't overlap anymore,
+ *      helps avoid a shift when preparing PD0 from PTE
+ *
+ * Vineetg: July 2009
+ *  -For MMU V2, we need not do heuristics at the time of commiting a D-TLB
+ *   entry, so that it doesn't knock out it's I-TLB entry
+ *  -Some more fine tuning:
+ *   bmsk instead of add, asl.cc instead of branch, delay slot utilise etc
+ *
+ * Vineetg: July 2009
+ *  -Practically rewrote the I/D TLB Miss handlers
+ *   Now 40 and 135 instructions a peice as compared to 131 and 449 resp.
+ *   Hence Leaner by 1.5 K
+ *   Used Conditional arithmetic to replace excessive branching
+ *   Also used short instructions wherever possible
+ *
+ * Vineetg: Aug 13th 2008
+ *  -Passing ECR (Exception Cause REG) to do_page_fault( ) for printing
+ *   more information in case of a Fatality
+ *
+ * Vineetg: March 25th Bug #92690
+ *  -Added Debug Code to check if sw-ASID == hw-ASID
+
+ * Rahul Trivedi, Amit Bhor: Codito Technologies 2004
+ */
+
+        .cpu A7
+
+#include <linux/linkage.h>
+#include <asm/entry.h>
+#include <asm/tlb.h>
+#include <asm/pgtable.h>
+#include <asm/arcregs.h>
+#include <asm/cache.h>
+#include <asm/processor.h>
+#if (CONFIG_ARC_MMU_VER == 1)
+#include <asm/tlb-mmu1.h>
+#endif
+
+;--------------------------------------------------------------------------
+; scratch memory to save the registers (r0-r3) used to code TLB refill Handler
+; For details refer to comments before TLBMISS_FREEUP_REGS below
+;--------------------------------------------------------------------------
+
+        .section .data
+        .global ex_saved_reg1
+        .align 1 << L1_CACHE_SHIFT      ; IMP: Must be Cache Line aligned
+        .type   ex_saved_reg1, @object
+        .size   ex_saved_reg1, 16
+ex_saved_reg1:
+        .zero 16
+
+;============================================================================
+;  Troubleshooting Stuff
+;============================================================================
+
+; Linux keeps ASID (Address Space ID) in task->active_mm->context.asid
+; When Creating TLB Entries, instead of doing 3 dependent loads from memory,
+; we use the MMU PID Reg to get current ASID.
+; In bizzare scenrios SW and HW ASID can get out-of-sync which is trouble.
+; So we try to detect this in TLB Mis shandler
+
+
+.macro DBG_ASID_MISMATCH
+
+#ifdef CONFIG_ARC_DBG_TLB_PARANOIA
+
+        ; make sure h/w ASID is same as s/w ASID
+
+        GET_CURR_TASK_ON_CPU  r3
+        ld r0, [r3, TASK_ACT_MM]
+        ld r0, [r0, MM_CTXT+MM_CTXT_ASID]
+
+        lr r1, [ARC_REG_PID]
+        and r1, r1, 0xFF
+        breq r1, r0, 5f
+
+        ; Error if H/w and S/w ASID don't match, but NOT if in kernel mode
+        lr  r0, [erstatus]
+        bbit0 r0, STATUS_U_BIT, 5f
+
+        ; We sure are in troubled waters, Flag the error, but to do so
+        ; need to switch to kernel mode stack to call error routine
+        GET_TSK_STACK_BASE   r3, sp
+
+        ; Call printk to shoutout aloud
+        mov r0, 1
+        j print_asid_mismatch
+
+5:   ; ASIDs match so proceed normally
+        nop
+
+#endif
+
+.endm
+
+;============================================================================
+;TLB Miss handling Code
+;============================================================================
+
+;-----------------------------------------------------------------------------
+; This macro does the page-table lookup for the faulting address.
+; OUT: r0 = PTE faulted on, r1 = ptr to PTE, r2 = Faulting V-address
+.macro LOAD_FAULT_PTE
+
+        lr  r2, [efa]
+
+        lr  r1, [ARC_REG_SCRATCH_DATA0] ; current pgd
+
+        lsr     r0, r2, PGDIR_SHIFT     ; Bits for indexing into PGD
+        ld.as   r1, [r1, r0]            ; PGD entry corresp to faulting addr
+        and.f   r1, r1, PAGE_MASK       ; Ignoring protection and other flags
+        ;   contains Ptr to Page Table
+        bz.d    do_slow_path_pf         ; if no Page Table, do page fault
+
+        ; Get the PTE entry: The idea is
+        ; (1) x = addr >> PAGE_SHIFT    -> masks page-off bits from @fault-addr
+        ; (2) y = x & (PTRS_PER_PTE - 1) -> to get index
+        ; (3) z = pgtbl[y]
+        ; To avoid the multiply by in end, we do the -2, <<2 below
+
+        lsr     r0, r2, (PAGE_SHIFT - 2)
+        and     r0, r0, ( (PTRS_PER_PTE - 1) << 2)
+        ld.aw   r0, [r1, r0]            ; get PTE and PTE ptr for fault addr
+
+.endm
+
+;-----------------------------------------------------------------
+; Convert Linux PTE entry into TLB entry
+; A one-word PTE entry is programmed as two-word TLB Entry [PD0:PD1] in mmu
+; IN: r0 = PTE, r1 = ptr to PTE
+
+.macro CONV_PTE_TO_TLB
+        and r3, r0, PTE_BITS_IN_PD1 ; Extract permission flags+PFN from PTE
+        sr  r3, [ARC_REG_TLBPD1]    ; these go in PD1
+
+        and r2, r0, PTE_BITS_IN_PD0 ; Extract other PTE flags: (V)alid, (G)lb
+#if (CONFIG_ARC_MMU_VER <= 2)   /* Neednot be done with v3 onwards */
+        lsr r2, r2                  ; shift PTE flags to match layout in PD0
+#endif
+
+        lr  r3,[ARC_REG_TLBPD0]     ; MMU prepares PD0 with vaddr and asid
+
+        or  r3, r3, r2              ; S | vaddr | {sasid|asid}
+        sr  r3,[ARC_REG_TLBPD0]     ; rewrite PD0
+.endm
+
+;-----------------------------------------------------------------
+; Commit the TLB entry into MMU
+
+.macro COMMIT_ENTRY_TO_MMU
+
+        /* Get free TLB slot: Set = computed from vaddr, way = random */
+        sr  TLBGetIndex, [ARC_REG_TLBCOMMAND]
+
+        /* Commit the Write */
+#if (CONFIG_ARC_MMU_VER >= 2)   /* introduced in v2 */
+        sr TLBWriteNI, [ARC_REG_TLBCOMMAND]
+#else
+        sr TLBWrite, [ARC_REG_TLBCOMMAND]
+#endif
+.endm
+
+;-----------------------------------------------------------------
+; ARC700 Exception Handling doesn't auto-switch stack and it only provides
+; ONE scratch AUX reg "ARC_REG_SCRATCH_DATA0"
+;
+; For Non-SMP, the scratch AUX reg is repurposed to cache task PGD, so a
+; "global" is used to free-up FIRST core reg to be able to code the rest of
+; exception prologue (IRQ auto-disabled on Exceptions, so it's IRQ-safe).
+; Since the Fast Path TLB Miss handler is coded with 4 regs, the remaining 3
+; need to be saved as well by extending the "global" to be 4 words. Hence
+;       ".size   ex_saved_reg1, 16"
+; [All of this dance is to avoid stack switching for each TLB Miss, since we
+; only need to save only a handful of regs, as opposed to complete reg file]
+
+; As simple as that....
+
+.macro TLBMISS_FREEUP_REGS
+        st    r0, [@ex_saved_reg1]
+        mov_s r0, @ex_saved_reg1
+        st_s  r1, [r0, 4]
+        st_s  r2, [r0, 8]
+        st_s  r3, [r0, 12]
+
+        ; VERIFY if the ASID in MMU-PID Reg is same as
+        ; one in Linux data structures
+
+        DBG_ASID_MISMATCH
+.endm
+
+;-----------------------------------------------------------------
+.macro TLBMISS_RESTORE_REGS
+        mov_s r0, @ex_saved_reg1
+        ld_s  r3, [r0,12]
+        ld_s  r2, [r0, 8]
+        ld_s  r1, [r0, 4]
+        ld_s  r0, [r0]
+.endm
+
+.section .text, "ax",@progbits  ;Fast Path Code, candidate for ICCM
+
+;-----------------------------------------------------------------------------
+; I-TLB Miss Exception Handler
+;-----------------------------------------------------------------------------
+
+ARC_ENTRY EV_TLBMissI
+
+        TLBMISS_FREEUP_REGS
+
+        ;----------------------------------------------------------------
+        ; Get the PTE corresponding to V-addr accessed
+        LOAD_FAULT_PTE
+
+        ;----------------------------------------------------------------
+        ; VERIFY_PTE: Check if PTE permissions approp for executing code
+        cmp_s   r2, VMALLOC_START
+        mov.lo  r2, (_PAGE_PRESENT | _PAGE_READ | _PAGE_EXECUTE)
+        mov.hs  r2, (_PAGE_PRESENT | _PAGE_K_READ | _PAGE_K_EXECUTE)
+
+        and     r3, r0, r2  ; Mask out NON Flag bits from PTE
+        xor.f   r3, r3, r2  ; check ( ( pte & flags_test ) == flags_test )
+        bnz     do_slow_path_pf
+
+        ; Let Linux VM know that the page was accessed
+        or      r0, r0, (_PAGE_PRESENT | _PAGE_ACCESSED)  ; set Accessed Bit
+        st_s    r0, [r1]                                  ; Write back PTE
+
+        CONV_PTE_TO_TLB
+        COMMIT_ENTRY_TO_MMU
+        TLBMISS_RESTORE_REGS
+        rtie
+
+ARC_EXIT EV_TLBMissI
+
+;-----------------------------------------------------------------------------
+; D-TLB Miss Exception Handler
+;-----------------------------------------------------------------------------
+
+ARC_ENTRY EV_TLBMissD
+
+        TLBMISS_FREEUP_REGS
+
+        ;----------------------------------------------------------------
+        ; Get the PTE corresponding to V-addr accessed
+        ; If PTE exists, it will setup, r0 = PTE, r1 = Ptr to PTE
+        LOAD_FAULT_PTE
+
+        ;----------------------------------------------------------------
+        ; VERIFY_PTE: Chk if PTE permissions approp for data access (R/W/R+W)
+
+        mov_s   r2, 0
+        lr      r3, [ecr]
+        btst_s  r3, ECR_C_BIT_DTLB_LD_MISS      ; Read Access
+        or.nz   r2, r2, _PAGE_READ              ; chk for Read flag in PTE
+        btst_s  r3, ECR_C_BIT_DTLB_ST_MISS      ; Write Access
+        or.nz   r2, r2, _PAGE_WRITE             ; chk for Write flag in PTE
+        ; Above laddering takes care of XCHG access
+        ;   which is both Read and Write
+
+        ; If kernel mode access, ; make _PAGE_xx flags as _PAGE_K_xx
+        ; For copy_(to|from)_user, despite exception taken in kernel mode,
+        ; this code is not hit, because EFA would still be the user mode
+        ; address (EFA < 0x6000_0000).
+        ; This code is for legit kernel mode faults, vmalloc specifically
+        ; (EFA: 0x7000_0000 to 0x7FFF_FFFF)
+
+        lr      r3, [efa]
+        cmp     r3, VMALLOC_START - 1   ; If kernel mode access
+        asl.hi  r2, r2, 3               ; make _PAGE_xx flags as _PAGE_K_xx
+        or      r2, r2, _PAGE_PRESENT   ; Common flag for K/U mode
+
+        ; By now, r2 setup with all the Flags we need to check in PTE
+        and     r3, r0, r2              ; Mask out NON Flag bits from PTE
+        brne.d  r3, r2, do_slow_path_pf ; is ((pte & flags_test) == flags_test)
+
+        ;----------------------------------------------------------------
+        ; UPDATE_PTE: Let Linux VM know that page was accessed/dirty
+        lr      r3, [ecr]
+        or      r0, r0, (_PAGE_PRESENT | _PAGE_ACCESSED) ; Accessed bit always
+        btst_s  r3,  ECR_C_BIT_DTLB_ST_MISS   ; See if it was a Write Access ?
+        or.nz   r0, r0, _PAGE_MODIFIED        ; if Write, set Dirty bit as well
+        st_s    r0, [r1]                      ; Write back PTE
+
+        CONV_PTE_TO_TLB
+
+#if (CONFIG_ARC_MMU_VER == 1)
+        ; MMU with 2 way set assoc J-TLB, needs some help in pathetic case of
+        ; memcpy where 3 parties contend for 2 ways, ensuing a livelock.
+        ; But only for old MMU or one with Metal Fix
+        TLB_WRITE_HEURISTICS
+#endif
+
+        COMMIT_ENTRY_TO_MMU
+        TLBMISS_RESTORE_REGS
+        rtie
+
+;-------- Common routine to call Linux Page Fault Handler -----------
+do_slow_path_pf:
+
+        ; Restore the 4-scratch regs saved by fast path miss handler
+        TLBMISS_RESTORE_REGS
+
+        ; Slow path TLB Miss handled as a regular ARC Exception
+        ; (stack switching / save the complete reg-file).
+        ; That requires freeing up r9
+        EXCPN_PROLOG_FREEUP_REG r9
+
+        lr  r9, [erstatus]
+
+        SWITCH_TO_KERNEL_STK
+        SAVE_ALL_SYS
+
+        ; ------- setup args for Linux Page fault Hanlder ---------
+        mov_s r0, sp
+        lr  r2, [efa]
+        lr  r3, [ecr]
+
+        ; Both st and ex imply WRITE access of some sort, hence do_page_fault( )
+        ; invoked with write=1 for DTLB-st/ex Miss and write=0 for ITLB miss or
+        ; DTLB-ld Miss
+        ; DTLB Miss Cause code is ld = 0x01 , st = 0x02, ex = 0x03
+        ; Following code uses that fact that st/ex have one bit in common
+
+        btst_s r3,  ECR_C_BIT_DTLB_ST_MISS
+        mov.z  r1, 0
+        mov.nz r1, 1
+
+        ; We don't want exceptions to be disabled while the fault is handled.
+        ; Now that we have saved the context we return from exception hence
+        ; exceptions get re-enable
+
+        FAKE_RET_FROM_EXCPN  r9
+
+        bl  do_page_fault
+        b   ret_from_exception
+
+ARC_EXIT EV_TLBMissD
+
+ARC_ENTRY EV_TLBMissB   ; Bogus entry to measure sz of DTLBMiss hdlr