arch/alpha/lib/ev6-clear_user.S

   1 /*
   2  * arch/alpha/lib/ev6-clear_user.S
   3  * 21264 version contributed by Rick Gorton <rick.gorton@alpha-processor.com>
   4  *
   5  * Zero user space, handling exceptions as we go.
   6  *
   7  * We have to make sure that $0 is always up-to-date and contains the
   8  * right "bytes left to zero" value (and that it is updated only _after_
   9  * a successful copy).  There is also some rather minor exception setup
  10  * stuff.
  11  *
  12  * Much of the information about 21264 scheduling/coding comes from:
  13  *      Compiler Writer's Guide for the Alpha 21264
  14  *      abbreviated as 'CWG' in other comments here
  15  *      ftp.digital.com/pub/Digital/info/semiconductor/literature/dsc-library.html
  16  * Scheduling notation:
  17  *      E       - either cluster
  18  *      U       - upper subcluster; U0 - subcluster U0; U1 - subcluster U1
  19  *      L       - lower subcluster; L0 - subcluster L0; L1 - subcluster L1
  20  * Try not to change the actual algorithm if possible for consistency.
  21  * Determining actual stalls (other than slotting) doesn't appear to be easy to do.
  22  * From perusing the source code context where this routine is called, it is
  23  * a fair assumption that significant fractions of entire pages are zeroed, so
  24  * it's going to be worth the effort to hand-unroll a big loop, and use wh64.
  25  * ASSUMPTION:
  26  *      The believed purpose of only updating $0 after a store is that a signal
  27  *      may come along during the execution of this chunk of code, and we don't
  28  *      want to leave a hole (and we also want to avoid repeating lots of work)
  29  */
  30
  31 #include <asm/export.h>
  32 /* Allow an exception for an insn; exit if we get one.  */
  33 #define EX(x,y...)                      \
  34         99: x,##y;                      \
  35         .section __ex_table,"a";        \
  36         .long 99b - .;                  \
  37         lda $31, $exception-99b($31);   \
  38         .previous
  39
  40         .set noat
  41         .set noreorder
  42         .align 4
  43
  44         .globl __clear_user
  45         .ent __clear_user
  46         .frame  $30, 0, $26
  47         .prologue 0
  48
  49                                 # Pipeline info : Slotting & Comments
  50 __clear_user:
  51         and     $17, $17, $0
  52         and     $16, 7, $4      # .. E  .. ..   : find dest head misalignment
  53         beq     $0, $zerolength # U  .. .. ..   :  U L U L
  54
  55         addq    $0, $4, $1      # .. .. .. E    : bias counter
  56         and     $1, 7, $2       # .. .. E  ..   : number of misaligned bytes in tail
  57 # Note - we never actually use $2, so this is a moot computation
  58 # and we can rewrite this later...
  59         srl     $1, 3, $1       # .. E  .. ..   : number of quadwords to clear
  60         beq     $4, $headalign  # U  .. .. ..   : U L U L
  61
  62 /*
  63  * Head is not aligned.  Write (8 - $4) bytes to head of destination
  64  * This means $16 is known to be misaligned
  65  */
  66         EX( ldq_u $5, 0($16) )  # .. .. .. L    : load dst word to mask back in
  67         beq     $1, $onebyte    # .. .. U  ..   : sub-word store?
  68         mskql   $5, $16, $5     # .. U  .. ..   : take care of misaligned head
  69         addq    $16, 8, $16     # E  .. .. ..   : L U U L
  70
  71         EX( stq_u $5, -8($16) ) # .. .. .. L    :
  72         subq    $1, 1, $1       # .. .. E  ..   :
  73         addq    $0, $4, $0      # .. E  .. ..   : bytes left -= 8 - misalignment
  74         subq    $0, 8, $0       # E  .. .. ..   : U L U L
  75
  76         .align  4
  77 /*
  78  * (The .align directive ought to be a moot point)
  79  * values upon initial entry to the loop
  80  * $1 is number of quadwords to clear (zero is a valid value)
  81  * $2 is number of trailing bytes (0..7) ($2 never used...)
  82  * $16 is known to be aligned 0mod8
  83  */
  84 $headalign:
  85         subq    $1, 16, $4      # .. .. .. E    : If < 16, we can not use the huge loop
  86         and     $16, 0x3f, $2   # .. .. E  ..   : Forward work for huge loop
  87         subq    $2, 0x40, $3    # .. E  .. ..   : bias counter (huge loop)
  88         blt     $4, $trailquad  # U  .. .. ..   : U L U L
  89
  90 /*
  91  * We know that we're going to do at least 16 quads, which means we are
  92  * going to be able to use the large block clear loop at least once.
  93  * Figure out how many quads we need to clear before we are 0mod64 aligned
  94  * so we can use the wh64 instruction.
  95  */
  96
  97         nop                     # .. .. .. E
  98         nop                     # .. .. E  ..
  99         nop                     # .. E  .. ..
 100         beq     $3, $bigalign   # U  .. .. ..   : U L U L : Aligned 0mod64
 101
 102 $alignmod64:
 103         EX( stq_u $31, 0($16) ) # .. .. .. L
 104         addq    $3, 8, $3       # .. .. E  ..
 105         subq    $0, 8, $0       # .. E  .. ..
 106         nop                     # E  .. .. ..   : U L U L
 107
 108         nop                     # .. .. .. E
 109         subq    $1, 1, $1       # .. .. E  ..
 110         addq    $16, 8, $16     # .. E  .. ..
 111         blt     $3, $alignmod64 # U  .. .. ..   : U L U L
 112
 113 $bigalign:
 114 /*
 115  * $0 is the number of bytes left
 116  * $1 is the number of quads left
 117  * $16 is aligned 0mod64
 118  * we know that we'll be taking a minimum of one trip through
 119  * CWG Section 3.7.6: do not expect a sustained store rate of > 1/cycle
 120  * We are _not_ going to update $0 after every single store.  That
 121  * would be silly, because there will be cross-cluster dependencies
 122  * no matter how the code is scheduled.  By doing it in slightly
 123  * staggered fashion, we can still do this loop in 5 fetches
 124  * The worse case will be doing two extra quads in some future execution,
 125  * in the event of an interrupted clear.
 126  * Assumes the wh64 needs to be for 2 trips through the loop in the future
 127  * The wh64 is issued on for the starting destination address for trip +2
 128  * through the loop, and if there are less than two trips left, the target
 129  * address will be for the current trip.
 130  */
 131         nop                     # E :
 132         nop                     # E :
 133         nop                     # E :
 134         bis     $16,$16,$3      # E : U L U L : Initial wh64 address is dest
 135         /* This might actually help for the current trip... */
 136
 137 $do_wh64:
 138         wh64    ($3)            # .. .. .. L1   : memory subsystem hint
 139         subq    $1, 16, $4      # .. .. E  ..   : Forward calculation - repeat the loop?
 140         EX( stq_u $31, 0($16) ) # .. L  .. ..
 141         subq    $0, 8, $0       # E  .. .. ..   : U L U L
 142
 143         addq    $16, 128, $3    # E : Target address of wh64
 144         EX( stq_u $31, 8($16) ) # L :
 145         EX( stq_u $31, 16($16) )        # L :
 146         subq    $0, 16, $0      # E : U L L U
 147
 148         nop                     # E :
 149         EX( stq_u $31, 24($16) )        # L :
 150         EX( stq_u $31, 32($16) )        # L :
 151         subq    $0, 168, $5     # E : U L L U : two trips through the loop left?
 152         /* 168 = 192 - 24, since we've already completed some stores */
 153
 154         subq    $0, 16, $0      # E :
 155         EX( stq_u $31, 40($16) )        # L :
 156         EX( stq_u $31, 48($16) )        # L :
 157         cmovlt  $5, $16, $3     # E : U L L U : Latency 2, extra mapping cycle
 158
 159         subq    $1, 8, $1       # E :
 160         subq    $0, 16, $0      # E :
 161         EX( stq_u $31, 56($16) )        # L :
 162         nop                     # E : U L U L
 163
 164         nop                     # E :
 165         subq    $0, 8, $0       # E :
 166         addq    $16, 64, $16    # E :
 167         bge     $4, $do_wh64    # U : U L U L
 168
 169 $trailquad:
 170         # zero to 16 quadwords left to store, plus any trailing bytes
 171         # $1 is the number of quadwords left to go.
 172         #
 173         nop                     # .. .. .. E
 174         nop                     # .. .. E  ..
 175         nop                     # .. E  .. ..
 176         beq     $1, $trailbytes # U  .. .. ..   : U L U L : Only 0..7 bytes to go
 177
 178 $onequad:
 179         EX( stq_u $31, 0($16) ) # .. .. .. L
 180         subq    $1, 1, $1       # .. .. E  ..
 181         subq    $0, 8, $0       # .. E  .. ..
 182         nop                     # E  .. .. ..   : U L U L
 183
 184         nop                     # .. .. .. E
 185         nop                     # .. .. E  ..
 186         addq    $16, 8, $16     # .. E  .. ..
 187         bgt     $1, $onequad    # U  .. .. ..   : U L U L
 188
 189         # We have an unknown number of bytes left to go.
 190 $trailbytes:
 191         nop                     # .. .. .. E
 192         nop                     # .. .. E  ..
 193         nop                     # .. E  .. ..
 194         beq     $0, $zerolength # U  .. .. ..   : U L U L
 195
 196         # $0 contains the number of bytes left to copy (0..31)
 197         # so we will use $0 as the loop counter
 198         # We know for a fact that $0 > 0 zero due to previous context
 199 $onebyte:
 200         EX( stb $31, 0($16) )   # .. .. .. L
 201         subq    $0, 1, $0       # .. .. E  ..   :
 202         addq    $16, 1, $16     # .. E  .. ..   :
 203         bgt     $0, $onebyte    # U  .. .. ..   : U L U L
 204
 205 $zerolength:
 206 $exception:                     # Destination for exception recovery(?)
 207         nop                     # .. .. .. E    :
 208         nop                     # .. .. E  ..   :
 209         nop                     # .. E  .. ..   :
 210         ret     $31, ($26), 1   # L0 .. .. ..   : L U L U
 211         .end __clear_user
 212         EXPORT_SYMBOL(__clear_user)