[Qemu-devel] Re: [PATCH 04/13] Respect length of watchpoints

[Jan Kiszka <jan.kiszka@web.de>]

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Glauber Costa wrote:
> On Tue, Oct 14, 2008 at 7:12 AM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
>> This adds length support for watchpoints. To keep things simple, only
>> aligned watchpoints are accepted.
> 
> why? It does not seem that much complicated to handle unaligned watchpoints.
> Unless I'm totally wrong, we should just store the value as-is, and
> then check for it.
> As a matter of fact, because we're masking and testing for the mask,
> it seems even more
> complicated to require that. I agree a full aligned world would be a
> happier world, but unfortunately,
> unaligned accesses are quite common in x86.

Unaligned watchpoints also means multi-page watchpoints - and this
introduces some complexity. I think the fact that real x86 hw
watchpoints require alignment as well motivated the simplification. But
if there is a real need for it (e.g. some other arch using the
infrastructure for hw watchpoint emulation), I could rethink this.
However, I would prefer to apply such extension on top of the proposed
implementation.

> 
>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>> ---
>>  cpu-defs.h |    2 +-
>>  exec.c     |   30 ++++++++++++++++++++----------
>>  2 files changed, 21 insertions(+), 11 deletions(-)
>>
>> Index: b/exec.c
>> ===================================================================
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -1313,14 +1313,21 @@ static void breakpoint_invalidate(CPUSta
>>  int cpu_watchpoint_insert(CPUState *env, target_ulong addr, target_ulong len,
>>                           int flags, CPUWatchpoint **watchpoint)
>>  {
>> +    target_ulong len_mask = ~(len - 1);
>>     CPUWatchpoint *wp;
>>
>> +    /* sanity checks: allow power-of-2 lengths, deny unaligned watchpoints */
>> +    if ((len != 1 && len != 2 && len != 4 && len != 8) || (addr & ~len_mask)) {
>> +        fprintf(stderr, "qemu: tried to set invalid watchpoint at "
>> +                TARGET_FMT_lx ", len=" TARGET_FMT_lu "\n", addr, len);
>> +        return -EINVAL;
>> +    }
>>     wp = qemu_malloc(sizeof(*wp));
>>     if (!wp)
>>         return -ENOBUFS;
>>
>>     wp->vaddr = addr;
>> -    wp->len = len;
>> +    wp->len_mask = len_mask;
>>     wp->flags = flags;
>>
>>     wp->next = env->watchpoints;
>> @@ -1344,10 +1351,12 @@ int cpu_watchpoint_insert(CPUState *env,
>>  int cpu_watchpoint_remove(CPUState *env, target_ulong addr, target_ulong len,
>>                           int flags)
>>  {
>> +    target_ulong len_mask = ~(len - 1);
>>     CPUWatchpoint *wp;
>>
>>     for (wp = env->watchpoints; wp != NULL; wp = wp->next) {
>> -        if (addr == wp->vaddr && len == wp->len && flags == wp->flags) {
>> +        if (addr == wp->vaddr && len_mask == wp->len_mask
>> +                && flags == wp->flags) {
>>             cpu_watchpoint_remove_by_ref(env, wp);
>>             return 0;
>>         }
>> @@ -2502,7 +2511,7 @@ static CPUWriteMemoryFunc *notdirty_mem_
>>  };
>>
>>  /* Generate a debug exception if a watchpoint has been hit.  */
>> -static void check_watchpoint(int offset, int flags)
>> +static void check_watchpoint(int offset, int len_mask, int flags)
>>  {
>>     CPUState *env = cpu_single_env;
>>     target_ulong vaddr;
>> @@ -2510,7 +2519,8 @@ static void check_watchpoint(int offset,
>>
>>     vaddr = (env->mem_io_vaddr & TARGET_PAGE_MASK) + offset;
>>     for (wp = env->watchpoints; wp != NULL; wp = wp->next) {
>> -        if (vaddr == wp->vaddr && (wp->flags & flags)) {
>> +        if ((vaddr == (wp->vaddr & len_mask) ||
>> +             (vaddr & wp->len_mask) == wp->vaddr) && (wp->flags & flags)) {
>>             env->watchpoint_hit = wp;
>>             cpu_interrupt(env, CPU_INTERRUPT_DEBUG);
>>             break;
>> @@ -2523,40 +2533,40 @@ static void check_watchpoint(int offset,
>>    phys routines.  */
>>  static uint32_t watch_mem_readb(void *opaque, target_phys_addr_t addr)
>>  {
>> -    check_watchpoint(addr & ~TARGET_PAGE_MASK, BP_MEM_READ);
>> +    check_watchpoint(addr & ~TARGET_PAGE_MASK, ~0x0, BP_MEM_READ);
>>     return ldub_phys(addr);
>>  }
>>
>>  static uint32_t watch_mem_readw(void *opaque, target_phys_addr_t addr)
>>  {
>> -    check_watchpoint(addr & ~TARGET_PAGE_MASK, BP_MEM_READ);
>> +    check_watchpoint(addr & ~TARGET_PAGE_MASK, ~0x1, BP_MEM_READ);
>>     return lduw_phys(addr);
>>  }
>>
>>  static uint32_t watch_mem_readl(void *opaque, target_phys_addr_t addr)
>>  {
>> -    check_watchpoint(addr & ~TARGET_PAGE_MASK, BP_MEM_READ);
>> +    check_watchpoint(addr & ~TARGET_PAGE_MASK, ~0x3, BP_MEM_READ);
>>     return ldl_phys(addr);
>>  }
>>
>>  static void watch_mem_writeb(void *opaque, target_phys_addr_t addr,
>>                              uint32_t val)
>>  {
>> -    check_watchpoint(addr & ~TARGET_PAGE_MASK, BP_MEM_WRITE);
>> +    check_watchpoint(addr & ~TARGET_PAGE_MASK, ~0x0, BP_MEM_WRITE);
>>     stb_phys(addr, val);
>>  }
>>
>>  static void watch_mem_writew(void *opaque, target_phys_addr_t addr,
>>                              uint32_t val)
>>  {
>> -    check_watchpoint(addr & ~TARGET_PAGE_MASK, BP_MEM_WRITE);
>> +    check_watchpoint(addr & ~TARGET_PAGE_MASK, ~0x1, BP_MEM_WRITE);
>>     stw_phys(addr, val);
>>  }
>>
>>  static void watch_mem_writel(void *opaque, target_phys_addr_t addr,
>>                              uint32_t val)
>>  {
>> -    check_watchpoint(addr & ~TARGET_PAGE_MASK, BP_MEM_WRITE);
>> +    check_watchpoint(addr & ~TARGET_PAGE_MASK, ~0x3, BP_MEM_WRITE);
>>     stl_phys(addr, val);
>>  }
>>
>> Index: b/cpu-defs.h
>> ===================================================================
>> --- a/cpu-defs.h
>> +++ b/cpu-defs.h
>> @@ -148,7 +148,7 @@ typedef struct CPUBreakpoint {
>>
>>  typedef struct CPUWatchpoint {
>>     target_ulong vaddr;
>> -    target_ulong len;
>> +    target_ulong len_mask;
>>     int flags; /* BP_* */
>>     struct CPUWatchpoint *prev, *next;
>>  } CPUWatchpoint;
> 
> It's less confusing if you call it len_mask from the beginning,
> instead of changing your own patch for that purpose.

OK. If I have to update the involved patches, I will merge this over.

Jan


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