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64b703ef27
linux/arch/arc/mm/tlbex.S
Vineet Gupta 64b703ef27 ARC: MMUv4 preps/1 - Fold PTE K/U access flags 
The current ARC VM code has 13 flags in Page Table entry: some software
(accesed/dirty/non-linear-maps) and rest hardware specific. With 8k MMU
page, we need 19 bits for addressing page frame so remaining 13 bits is
just about enough to accomodate the current flags.

In MMUv4 there are 2 additional flags, SZ (normal or super page) and WT
(cache access mode write-thru) - and additionally PFN is 20 bits (vs. 19
before for 8k). Thus these can't be held in current PTE w/o making each
entry 64bit wide.

It seems there is some scope of compressing the current PTE flags (and
freeing up a few bits). Currently PTE contains fully orthogonal distinct
access permissions for kernel and user mode (Kr, Kw, Kx; Ur, Uw, Ux)
which can be folded into one set (R, W, X). The translation of 3 PTE
bits into 6 TLB bits (when programming the MMU) can be done based on
following pre-requites/assumptions:

1. For kernel-mode-only translations (vmalloc: 0x7000_0000 to
   0x7FFF_FFFF), PTE additionally has PAGE_GLOBAL flag set (and user
   space entries can never be global). Thus such a PTE can translate
   to Kr, Kw, Kx (as appropriate) and zero for User mode counterparts.

2. For non global entries, the PTE flags can be used to create mirrored
   K and U TLB bits. This is true after commit a950549c67
   "ARC: copy_(to|from)_user() to honor usermode-access permissions"
   which ensured that user-space translations _MUST_ have same access
   permissions for both U/K mode accesses so that  copy_{to,from}_user()
   play fair with fault based CoW break and such...

There is no such thing as free lunch - the cost is slightly infalted
TLB-Miss Handlers.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
2013-08-29 17:51:36 +05:30

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/*
* 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/mmu.h>
#include <asm/pgtable.h>
#include <asm/arcregs.h>
#include <asm/cache.h>
#include <asm/processor.h>
#include <asm/tlb-mmu1.h>
;-----------------------------------------------------------------
; 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]
;
; For ARC700 SMP, the "global" obviously can't be used for free up the FIRST
; core reg as it will not be SMP safe.
; Thus scratch AUX reg is used (and no longer used to cache task PGD).
; To save the rest of 3 regs - per cpu, the global is made "per-cpu".
; Epilogue thus has to locate the "per-cpu" storage for regs.
; To avoid cache line bouncing the per-cpu global is aligned/sized per
; L1_CACHE_SHIFT, despite fundamentally needing to be 12 bytes only. Hence
; ".size ex_saved_reg1, (CONFIG_NR_CPUS << L1_CACHE_SHIFT)"
; As simple as that....
;--------------------------------------------------------------------------
; scratch memory to save [r0-r3] used to code TLB refill Handler
ARCFP_DATA ex_saved_reg1
.align 1 << L1_CACHE_SHIFT
.type ex_saved_reg1, @object
#ifdef CONFIG_SMP
.size ex_saved_reg1, (CONFIG_NR_CPUS << L1_CACHE_SHIFT)
ex_saved_reg1:
.zero (CONFIG_NR_CPUS << L1_CACHE_SHIFT)
#else
.size ex_saved_reg1, 16
ex_saved_reg1:
.zero 16
#endif
.macro TLBMISS_FREEUP_REGS
#ifdef CONFIG_SMP
sr r0, [ARC_REG_SCRATCH_DATA0] ; freeup r0 to code with
GET_CPU_ID r0 ; get to per cpu scratch mem,
lsl r0, r0, L1_CACHE_SHIFT ; cache line wide per cpu
add r0, @ex_saved_reg1, r0
#else
st r0, [@ex_saved_reg1]
mov_s r0, @ex_saved_reg1
#endif
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
#ifdef CONFIG_SMP
GET_CPU_ID r0 ; get to per cpu scratch mem
lsl r0, r0, L1_CACHE_SHIFT ; each is cache line wide
add r0, @ex_saved_reg1, r0
ld_s r3, [r0,12]
ld_s r2, [r0, 8]
ld_s r1, [r0, 4]
lr r0, [ARC_REG_SCRATCH_DATA0]
#else
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]
#endif
.endm
;============================================================================
; 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]
#ifndef CONFIG_SMP
lr r1, [ARC_REG_SCRATCH_DATA0] ; current pgd
#else
GET_CURR_TASK_ON_CPU r1
ld r1, [r1, TASK_ACT_MM]
ld r1, [r1, MM_PGD]
#endif
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
#ifdef CONFIG_ARC_DBG_TLB_MISS_COUNT
and.f 0, r0, _PAGE_PRESENT
bz 1f
ld r3, [num_pte_not_present]
add r3, r3, 1
st r3, [num_pte_not_present]
1:
#endif
.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_RWX ; r w x
lsl r2, r3, 3 ; r w x 0 0 0
and.f 0, r0, _PAGE_GLOBAL
or.z r2, r2, r3 ; r w x r w x
and r3, r0, PTE_BITS_NON_RWX_IN_PD1 ; Extract PFN+cache bits from PTE
or r3, r3, r2
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
ARCFP_CODE ;Fast Path Code, candidate for ICCM
;-----------------------------------------------------------------------------
; I-TLB Miss Exception Handler
;-----------------------------------------------------------------------------
ARC_ENTRY EV_TLBMissI
TLBMISS_FREEUP_REGS
#ifdef CONFIG_ARC_DBG_TLB_MISS_COUNT
ld r0, [@numitlb]
add r0, r0, 1
st r0, [@numitlb]
#endif
;----------------------------------------------------------------
; Get the PTE corresponding to V-addr accessed, r2 is setup with EFA
LOAD_FAULT_PTE
;----------------------------------------------------------------
; VERIFY_PTE: Check if PTE permissions approp for executing code
cmp_s r2, VMALLOC_START
mov_s r2, (_PAGE_PRESENT | _PAGE_EXECUTE)
or.hs r2, r2, _PAGE_GLOBAL
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_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
#ifdef CONFIG_ARC_DBG_TLB_MISS_COUNT
ld r0, [@numdtlb]
add r0, r0, 1
st r0, [@numdtlb]
#endif
;----------------------------------------------------------------
; Get the PTE corresponding to V-addr accessed
; If PTE exists, it will setup, r0 = PTE, r1 = Ptr to PTE, r2 = EFA
LOAD_FAULT_PTE
;----------------------------------------------------------------
; VERIFY_PTE: Chk if PTE permissions approp for data access (R/W/R+W)
cmp_s r2, VMALLOC_START
mov_s r2, _PAGE_PRESENT ; common bit for K/U PTE
or.hs r2, r2, _PAGE_GLOBAL ; kernel PTE only
; Linux PTE [RWX] bits are semantically overloaded:
; -If PAGE_GLOBAL set, they refer to kernel-only flags (vmalloc)
; -Otherwise they are user-mode permissions, and those are exactly
; same for kernel mode as well (e.g. copy_(to|from)_user)
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 (both R and W)
; 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_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).
EXCEPTION_PROLOGUE
; ------- setup args for Linux Page fault Hanlder ---------
mov_s r0, sp
lr r1, [efa]
; 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
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