aarch64-linux-hw-point.c

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/* Copyright (C) 2009-2018 Free Software Foundation, Inc.
   Contributed by ARM Ltd.

   This file is part of GDB.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

#include "common-defs.h"
#include "break-common.h"
#include "common-regcache.h"
#include "nat/linux-nat.h"
#include "aarch64-linux-hw-point.h"

#include <sys/uio.h>
#include <asm/ptrace.h>
#include <sys/ptrace.h>
#include <elf.h>

/* Number of hardware breakpoints/watchpoints the target supports.
   They are initialized with values obtained via the ptrace calls
   with NT_ARM_HW_BREAK and NT_ARM_HW_WATCH respectively.  */

int aarch64_num_bp_regs;
int aarch64_num_wp_regs;

/* Utility function that returns the length in bytes of a watchpoint
   according to the content of a hardware debug control register CTRL.
   Note that the kernel currently only supports the following Byte
   Address Select (BAS) values: 0x1, 0x3, 0xf and 0xff, which means
   that for a hardware watchpoint, its valid length can only be 1
   byte, 2 bytes, 4 bytes or 8 bytes.  */

unsigned int
aarch64_watchpoint_length (unsigned int ctrl)
{
  switch (DR_CONTROL_LENGTH (ctrl))
    {
    case 0x01:
      return 1;
    case 0x03:
      return 2;
    case 0x0f:
      return 4;
    case 0xff:
      return 8;
    default:
      return 0;
    }
}

/* Given the hardware breakpoint or watchpoint type TYPE and its
   length LEN, return the expected encoding for a hardware
   breakpoint/watchpoint control register.  */

static unsigned int
aarch64_point_encode_ctrl_reg (enum target_hw_bp_type type, int len)
{
  unsigned int ctrl, ttype;

  /* type */
  switch (type)
    {
    case hw_write:
      ttype = 2;
      break;
    case hw_read:
      ttype = 1;
      break;
    case hw_access:
      ttype = 3;
      break;
    case hw_execute:
      ttype = 0;
      break;
    default:
      perror_with_name (_("Unrecognized breakpoint/watchpoint type"));
    }

  ctrl = ttype << 3;

  /* length bitmask */
  ctrl |= ((1 << len) - 1) << 5;
  /* enabled at el0 */
  ctrl |= (2 << 1) | 1;

  return ctrl;
}

/* Addresses to be written to the hardware breakpoint and watchpoint
   value registers need to be aligned; the alignment is 4-byte and
   8-type respectively.  Linux kernel rejects any non-aligned address
   it receives from the related ptrace call.  Furthermore, the kernel
   currently only supports the following Byte Address Select (BAS)
   values: 0x1, 0x3, 0xf and 0xff, which means that for a hardware
   watchpoint to be accepted by the kernel (via ptrace call), its
   valid length can only be 1 byte, 2 bytes, 4 bytes or 8 bytes.
   Despite these limitations, the unaligned watchpoint is supported in
   this port.

   Return 0 for any non-compliant ADDR and/or LEN; return 1 otherwise.  */

static int
aarch64_point_is_aligned (int is_watchpoint, CORE_ADDR addr, int len)
{
  unsigned int alignment = 0;

  if (is_watchpoint)
    alignment = AARCH64_HWP_ALIGNMENT;
  else
    {
      struct regcache *regcache
    = get_thread_regcache_for_ptid (current_lwp_ptid ());

      /* Set alignment to 2 only if the current process is 32-bit,
     since thumb instruction can be 2-byte aligned.  Otherwise, set
     alignment to AARCH64_HBP_ALIGNMENT.  */
      if (regcache_register_size (regcache, 0) == 8)
    alignment = AARCH64_HBP_ALIGNMENT;
      else
    alignment = 2;
    }

  if (addr & (alignment - 1))
    return 0;

  if (len != 8 && len != 4 && len != 2 && len != 1)
    return 0;

  return 1;
}

/* Given the (potentially unaligned) watchpoint address in ADDR and
   length in LEN, return the aligned address and aligned length in
   *ALIGNED_ADDR_P and *ALIGNED_LEN_P, respectively.  The returned
   aligned address and length will be valid values to write to the
   hardware watchpoint value and control registers.

   The given watchpoint may get truncated if more than one hardware
   register is needed to cover the watched region.  *NEXT_ADDR_P
   and *NEXT_LEN_P, if non-NULL, will return the address and length
   of the remaining part of the watchpoint (which can be processed
   by calling this routine again to generate another aligned address
   and length pair.

   Essentially, unaligned watchpoint is achieved by minimally
   enlarging the watched area to meet the alignment requirement, and
   if necessary, splitting the watchpoint over several hardware
   watchpoint registers.  The trade-off is that there will be
   false-positive hits for the read-type or the access-type hardware
   watchpoints; for the write type, which is more commonly used, there
   will be no such issues, as the higher-level breakpoint management
   in gdb always examines the exact watched region for any content
   change, and transparently resumes a thread from a watchpoint trap
   if there is no change to the watched region.

   Another limitation is that because the watched region is enlarged,
   the watchpoint fault address returned by
   aarch64_stopped_data_address may be outside of the original watched
   region, especially when the triggering instruction is accessing a
   larger region.  When the fault address is not within any known
   range, watchpoints_triggered in gdb will get confused, as the
   higher-level watchpoint management is only aware of original
   watched regions, and will think that some unknown watchpoint has
   been triggered.  In such a case, gdb may stop without displaying
   any detailed information.

   Once the kernel provides the full support for Byte Address Select
   (BAS) in the hardware watchpoint control register, these
   limitations can be largely relaxed with some further work.  */

static void
aarch64_align_watchpoint (CORE_ADDR addr, int len, CORE_ADDR *aligned_addr_p,
              int *aligned_len_p, CORE_ADDR *next_addr_p,
              int *next_len_p)
{
  int aligned_len;
  unsigned int offset;
  CORE_ADDR aligned_addr;
  const unsigned int alignment = AARCH64_HWP_ALIGNMENT;
  const unsigned int max_wp_len = AARCH64_HWP_MAX_LEN_PER_REG;

  /* As assumed by the algorithm.  */
  gdb_assert (alignment == max_wp_len);

  if (len <= 0)
    return;

  /* Address to be put into the hardware watchpoint value register
     must be aligned.  */
  offset = addr & (alignment - 1);
  aligned_addr = addr - offset;

  gdb_assert (offset >= 0 && offset < alignment);
  gdb_assert (aligned_addr >= 0 && aligned_addr <= addr);
  gdb_assert (offset + len > 0);

  if (offset + len >= max_wp_len)
    {
      /* Need more than one watchpoint registers; truncate it at the
     alignment boundary.  */
      aligned_len = max_wp_len;
      len -= (max_wp_len - offset);
      addr += (max_wp_len - offset);
      gdb_assert ((addr & (alignment - 1)) == 0);
    }
  else
    {
      /* Find the smallest valid length that is large enough to
     accommodate this watchpoint.  */
      static const unsigned char
    aligned_len_array[AARCH64_HWP_MAX_LEN_PER_REG] =
    { 1, 2, 4, 4, 8, 8, 8, 8 };

      aligned_len = aligned_len_array[offset + len - 1];
      addr += len;
      len = 0;
    }

  if (aligned_addr_p)
    *aligned_addr_p = aligned_addr;
  if (aligned_len_p)
    *aligned_len_p = aligned_len;
  if (next_addr_p)
    *next_addr_p = addr;
  if (next_len_p)
    *next_len_p = len;
}

struct aarch64_dr_update_callback_param
{
  int is_watchpoint;
  unsigned int idx;
};

/* Callback for iterate_over_lwps.  Records the
   information about the change of one hardware breakpoint/watchpoint
   setting for the thread LWP.
   The information is passed in via PTR.
   N.B.  The actual updating of hardware debug registers is not
   carried out until the moment the thread is resumed.  */

static int
debug_reg_change_callback (struct lwp_info *lwp, void *ptr)
{
  struct aarch64_dr_update_callback_param *param_p
    = (struct aarch64_dr_update_callback_param *) ptr;
  int tid = ptid_get_lwp (ptid_of_lwp (lwp));
  int idx = param_p->idx;
  int is_watchpoint = param_p->is_watchpoint;
  struct arch_lwp_info *info = lwp_arch_private_info (lwp);
  dr_changed_t *dr_changed_ptr;
  dr_changed_t dr_changed;

  if (info == NULL)
    {
      info = XCNEW (struct arch_lwp_info);
      lwp_set_arch_private_info (lwp, info);
    }

  if (show_debug_regs)
    {
      debug_printf ("debug_reg_change_callback: \n\tOn entry:\n");
      debug_printf ("\ttid%d, dr_changed_bp=0x%s, "
            "dr_changed_wp=0x%s\n", tid,
            phex (info->dr_changed_bp, 8),
            phex (info->dr_changed_wp, 8));
    }

  dr_changed_ptr = is_watchpoint ? &info->dr_changed_wp
    : &info->dr_changed_bp;
  dr_changed = *dr_changed_ptr;

  gdb_assert (idx >= 0
          && (idx <= (is_watchpoint ? aarch64_num_wp_regs
              : aarch64_num_bp_regs)));

  /* The actual update is done later just before resuming the lwp,
     we just mark that one register pair needs updating.  */
  DR_MARK_N_CHANGED (dr_changed, idx);
  *dr_changed_ptr = dr_changed;

  /* If the lwp isn't stopped, force it to momentarily pause, so
     we can update its debug registers.  */
  if (!lwp_is_stopped (lwp))
    linux_stop_lwp (lwp);

  if (show_debug_regs)
    {
      debug_printf ("\tOn exit:\n\ttid%d, dr_changed_bp=0x%s, "
            "dr_changed_wp=0x%s\n", tid,
            phex (info->dr_changed_bp, 8),
            phex (info->dr_changed_wp, 8));
    }

  return 0;
}

/* Notify each thread that their IDXth breakpoint/watchpoint register
   pair needs to be updated.  The message will be recorded in each
   thread's arch-specific data area, the actual updating will be done
   when the thread is resumed.  */

static void
aarch64_notify_debug_reg_change (const struct aarch64_debug_reg_state *state,
                 int is_watchpoint, unsigned int idx)
{
  struct aarch64_dr_update_callback_param param;
  ptid_t pid_ptid = pid_to_ptid (ptid_get_pid (current_lwp_ptid ()));

  param.is_watchpoint = is_watchpoint;
  param.idx = idx;

  iterate_over_lwps (pid_ptid, debug_reg_change_callback, (void *) &param);
}

/* Record the insertion of one breakpoint/watchpoint, as represented
   by ADDR and CTRL, in the process' arch-specific data area *STATE.  */

static int
aarch64_dr_state_insert_one_point (struct aarch64_debug_reg_state *state,
                   enum target_hw_bp_type type,
                   CORE_ADDR addr, int len)
{
  int i, idx, num_regs, is_watchpoint;
  unsigned int ctrl, *dr_ctrl_p, *dr_ref_count;
  CORE_ADDR *dr_addr_p;

  /* Set up state pointers.  */
  is_watchpoint = (type != hw_execute);
  gdb_assert (aarch64_point_is_aligned (is_watchpoint, addr, len));
  if (is_watchpoint)
    {
      num_regs = aarch64_num_wp_regs;
      dr_addr_p = state->dr_addr_wp;
      dr_ctrl_p = state->dr_ctrl_wp;
      dr_ref_count = state->dr_ref_count_wp;
    }
  else
    {
      num_regs = aarch64_num_bp_regs;
      dr_addr_p = state->dr_addr_bp;
      dr_ctrl_p = state->dr_ctrl_bp;
      dr_ref_count = state->dr_ref_count_bp;
    }

  ctrl = aarch64_point_encode_ctrl_reg (type, len);

  /* Find an existing or free register in our cache.  */
  idx = -1;
  for (i = 0; i < num_regs; ++i)
    {
      if ((dr_ctrl_p[i] & 1) == 0)
    {
      gdb_assert (dr_ref_count[i] == 0);
      idx = i;
      /* no break; continue hunting for an exising one.  */
    }
      else if (dr_addr_p[i] == addr && dr_ctrl_p[i] == ctrl)
    {
      gdb_assert (dr_ref_count[i] != 0);
      idx = i;
      break;
    }
    }

  /* No space.  */
  if (idx == -1)
    return -1;

  /* Update our cache.  */
  if ((dr_ctrl_p[idx] & 1) == 0)
    {
      /* new entry */
      dr_addr_p[idx] = addr;
      dr_ctrl_p[idx] = ctrl;
      dr_ref_count[idx] = 1;
      /* Notify the change.  */
      aarch64_notify_debug_reg_change (state, is_watchpoint, idx);
    }
  else
    {
      /* existing entry */
      dr_ref_count[idx]++;
    }

  return 0;
}

/* Record the removal of one breakpoint/watchpoint, as represented by
   ADDR and CTRL, in the process' arch-specific data area *STATE.  */

static int
aarch64_dr_state_remove_one_point (struct aarch64_debug_reg_state *state,
                   enum target_hw_bp_type type,
                   CORE_ADDR addr, int len)
{
  int i, num_regs, is_watchpoint;
  unsigned int ctrl, *dr_ctrl_p, *dr_ref_count;
  CORE_ADDR *dr_addr_p;

  /* Set up state pointers.  */
  is_watchpoint = (type != hw_execute);
  if (is_watchpoint)
    {
      num_regs = aarch64_num_wp_regs;
      dr_addr_p = state->dr_addr_wp;
      dr_ctrl_p = state->dr_ctrl_wp;
      dr_ref_count = state->dr_ref_count_wp;
    }
  else
    {
      num_regs = aarch64_num_bp_regs;
      dr_addr_p = state->dr_addr_bp;
      dr_ctrl_p = state->dr_ctrl_bp;
      dr_ref_count = state->dr_ref_count_bp;
    }

  ctrl = aarch64_point_encode_ctrl_reg (type, len);

  /* Find the entry that matches the ADDR and CTRL.  */
  for (i = 0; i < num_regs; ++i)
    if (dr_addr_p[i] == addr && dr_ctrl_p[i] == ctrl)
      {
    gdb_assert (dr_ref_count[i] != 0);
    break;
      }

  /* Not found.  */
  if (i == num_regs)
    return -1;

  /* Clear our cache.  */
  if (--dr_ref_count[i] == 0)
    {
      /* Clear the enable bit.  */
      ctrl &= ~1;
      dr_addr_p[i] = 0;
      dr_ctrl_p[i] = ctrl;
      /* Notify the change.  */
      aarch64_notify_debug_reg_change (state, is_watchpoint, i);
    }

  return 0;
}

int
aarch64_handle_breakpoint (enum target_hw_bp_type type, CORE_ADDR addr,
               int len, int is_insert,
               struct aarch64_debug_reg_state *state)
{
  if (is_insert)
    {
      /* The hardware breakpoint on AArch64 should always be 4-byte
     aligned, but on AArch32, it can be 2-byte aligned.  Note that
     we only check the alignment on inserting breakpoint because
     aarch64_point_is_aligned needs the inferior_ptid inferior's
     regcache to decide whether the inferior is 32-bit or 64-bit.
     However when GDB follows the parent process and detach breakpoints
     from child process, inferior_ptid is the child ptid, but the
     child inferior doesn't exist in GDB's view yet.  */
      if (!aarch64_point_is_aligned (0 /* is_watchpoint */ , addr, len))
    return -1;

      return aarch64_dr_state_insert_one_point (state, type, addr, len);
    }
  else
    return aarch64_dr_state_remove_one_point (state, type, addr, len);
}

/* This is essentially the same as aarch64_handle_breakpoint, apart
   from that it is an aligned watchpoint to be handled.  */

static int
aarch64_handle_aligned_watchpoint (enum target_hw_bp_type type,
                   CORE_ADDR addr, int len, int is_insert,
                   struct aarch64_debug_reg_state *state)
{
  if (is_insert)
    return aarch64_dr_state_insert_one_point (state, type, addr, len);
  else
    return aarch64_dr_state_remove_one_point (state, type, addr, len);
}

/* Insert/remove unaligned watchpoint by calling
   aarch64_align_watchpoint repeatedly until the whole watched region,
   as represented by ADDR and LEN, has been properly aligned and ready
   to be written to one or more hardware watchpoint registers.
   IS_INSERT indicates whether this is an insertion or a deletion.
   Return 0 if succeed.  */

static int
aarch64_handle_unaligned_watchpoint (enum target_hw_bp_type type,
                     CORE_ADDR addr, int len, int is_insert,
                     struct aarch64_debug_reg_state *state)
{
  while (len > 0)
    {
      CORE_ADDR aligned_addr;
      int aligned_len, ret;

      aarch64_align_watchpoint (addr, len, &aligned_addr, &aligned_len,
                &addr, &len);

      if (is_insert)
    ret = aarch64_dr_state_insert_one_point (state, type, aligned_addr,
                         aligned_len);
      else
    ret = aarch64_dr_state_remove_one_point (state, type, aligned_addr,
                         aligned_len);

      if (show_debug_regs)
    debug_printf ("handle_unaligned_watchpoint: is_insert: %d\n"
              "                             "
              "aligned_addr: %s, aligned_len: %d\n"
              "                                "
              "next_addr: %s,    next_len: %d\n",
              is_insert, core_addr_to_string_nz (aligned_addr),
              aligned_len, core_addr_to_string_nz (addr), len);

      if (ret != 0)
    return ret;
    }

  return 0;
}

int
aarch64_handle_watchpoint (enum target_hw_bp_type type, CORE_ADDR addr,
               int len, int is_insert,
               struct aarch64_debug_reg_state *state)
{
  if (aarch64_point_is_aligned (1 /* is_watchpoint */ , addr, len))
    return aarch64_handle_aligned_watchpoint (type, addr, len, is_insert,
                          state);
  else
    return aarch64_handle_unaligned_watchpoint (type, addr, len, is_insert,
                        state);
}

/* Call ptrace to set the thread TID's hardware breakpoint/watchpoint
   registers with data from *STATE.  */

void
aarch64_linux_set_debug_regs (const struct aarch64_debug_reg_state *state,
                  int tid, int watchpoint)
{
  int i, count;
  struct iovec iov;
  struct user_hwdebug_state regs;
  const CORE_ADDR *addr;
  const unsigned int *ctrl;

  memset (&regs, 0, sizeof (regs));
  iov.iov_base = &regs;
  count = watchpoint ? aarch64_num_wp_regs : aarch64_num_bp_regs;
  addr = watchpoint ? state->dr_addr_wp : state->dr_addr_bp;
  ctrl = watchpoint ? state->dr_ctrl_wp : state->dr_ctrl_bp;
  if (count == 0)
    return;
  iov.iov_len = (offsetof (struct user_hwdebug_state, dbg_regs)
         + count * sizeof (regs.dbg_regs[0]));

  for (i = 0; i < count; i++)
    {
      regs.dbg_regs[i].addr = addr[i];
      regs.dbg_regs[i].ctrl = ctrl[i];
    }

  if (ptrace (PTRACE_SETREGSET, tid,
          watchpoint ? NT_ARM_HW_WATCH : NT_ARM_HW_BREAK,
          (void *) &iov))
    error (_("Unexpected error setting hardware debug registers"));
}

/* Print the values of the cached breakpoint/watchpoint registers.  */

void
aarch64_show_debug_reg_state (struct aarch64_debug_reg_state *state,
                  const char *func, CORE_ADDR addr,
                  int len, enum target_hw_bp_type type)
{
  int i;

  debug_printf ("%s", func);
  if (addr || len)
    debug_printf (" (addr=0x%08lx, len=%d, type=%s)",
          (unsigned long) addr, len,
          type == hw_write ? "hw-write-watchpoint"
          : (type == hw_read ? "hw-read-watchpoint"
             : (type == hw_access ? "hw-access-watchpoint"
            : (type == hw_execute ? "hw-breakpoint"
               : "??unknown??"))));
  debug_printf (":\n");

  debug_printf ("\tBREAKPOINTs:\n");
  for (i = 0; i < aarch64_num_bp_regs; i++)
    debug_printf ("\tBP%d: addr=%s, ctrl=0x%08x, ref.count=%d\n",
          i, core_addr_to_string_nz (state->dr_addr_bp[i]),
          state->dr_ctrl_bp[i], state->dr_ref_count_bp[i]);

  debug_printf ("\tWATCHPOINTs:\n");
  for (i = 0; i < aarch64_num_wp_regs; i++)
    debug_printf ("\tWP%d: addr=%s, ctrl=0x%08x, ref.count=%d\n",
          i, core_addr_to_string_nz (state->dr_addr_wp[i]),
          state->dr_ctrl_wp[i], state->dr_ref_count_wp[i]);
}

/* Get the hardware debug register capacity information from the
   process represented by TID.  */

void
aarch64_linux_get_debug_reg_capacity (int tid)
{
  struct iovec iov;
  struct user_hwdebug_state dreg_state;

  iov.iov_base = &dreg_state;
  iov.iov_len = sizeof (dreg_state);

  /* Get hardware watchpoint register info.  */
  if (ptrace (PTRACE_GETREGSET, tid, NT_ARM_HW_WATCH, &iov) == 0
      && (AARCH64_DEBUG_ARCH (dreg_state.dbg_info) == AARCH64_DEBUG_ARCH_V8
      || AARCH64_DEBUG_ARCH (dreg_state.dbg_info) == AARCH64_DEBUG_ARCH_V8_1
      || AARCH64_DEBUG_ARCH (dreg_state.dbg_info) == AARCH64_DEBUG_ARCH_V8_2))
    {
      aarch64_num_wp_regs = AARCH64_DEBUG_NUM_SLOTS (dreg_state.dbg_info);
      if (aarch64_num_wp_regs > AARCH64_HWP_MAX_NUM)
    {
      warning (_("Unexpected number of hardware watchpoint registers"
             " reported by ptrace, got %d, expected %d."),
           aarch64_num_wp_regs, AARCH64_HWP_MAX_NUM);
      aarch64_num_wp_regs = AARCH64_HWP_MAX_NUM;
    }
    }
  else
    {
      warning (_("Unable to determine the number of hardware watchpoints"
         " available."));
      aarch64_num_wp_regs = 0;
    }

  /* Get hardware breakpoint register info.  */
  if (ptrace (PTRACE_GETREGSET, tid, NT_ARM_HW_BREAK, &iov) == 0
      && (AARCH64_DEBUG_ARCH (dreg_state.dbg_info) == AARCH64_DEBUG_ARCH_V8
      || AARCH64_DEBUG_ARCH (dreg_state.dbg_info) == AARCH64_DEBUG_ARCH_V8_1
      || AARCH64_DEBUG_ARCH (dreg_state.dbg_info) == AARCH64_DEBUG_ARCH_V8_2))
    {
      aarch64_num_bp_regs = AARCH64_DEBUG_NUM_SLOTS (dreg_state.dbg_info);
      if (aarch64_num_bp_regs > AARCH64_HBP_MAX_NUM)
    {
      warning (_("Unexpected number of hardware breakpoint registers"
             " reported by ptrace, got %d, expected %d."),
           aarch64_num_bp_regs, AARCH64_HBP_MAX_NUM);
      aarch64_num_bp_regs = AARCH64_HBP_MAX_NUM;
    }
    }
  else
    {
      warning (_("Unable to determine the number of hardware breakpoints"
         " available."));
      aarch64_num_bp_regs = 0;
    }
}

/* Return true if we can watch a memory region that starts address
   ADDR and whose length is LEN in bytes.  */

int
aarch64_linux_region_ok_for_watchpoint (CORE_ADDR addr, int len)
{
  CORE_ADDR aligned_addr;

  /* Can not set watchpoints for zero or negative lengths.  */
  if (len <= 0)
    return 0;

  /* Must have hardware watchpoint debug register(s).  */
  if (aarch64_num_wp_regs == 0)
    return 0;

  /* We support unaligned watchpoint address and arbitrary length,
     as long as the size of the whole watched area after alignment
     doesn't exceed size of the total area that all watchpoint debug
     registers can watch cooperatively.

     This is a very relaxed rule, but unfortunately there are
     limitations, e.g. false-positive hits, due to limited support of
     hardware debug registers in the kernel.  See comment above
     aarch64_align_watchpoint for more information.  */

  aligned_addr = addr & ~(AARCH64_HWP_MAX_LEN_PER_REG - 1);
  if (aligned_addr + aarch64_num_wp_regs * AARCH64_HWP_MAX_LEN_PER_REG
      < addr + len)
    return 0;

  /* All tests passed so we are likely to be able to set the watchpoint.
     The reason that it is 'likely' rather than 'must' is because
     we don't check the current usage of the watchpoint registers, and
     there may not be enough registers available for this watchpoint.
     Ideally we should check the cached debug register state, however
     the checking is costly.  */
  return 1;
}