Allow probing of a watchpoint *without* raising an exception.
This is of most use for no-fault loads, which should indicate
via some architectural means that the load did not occur.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 include/hw/core/cpu.h |  7 +++++++
 exec.c                | 19 +++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
index 73e9a869a4..8ec44267a4 100644
--- a/include/hw/core/cpu.h
+++ b/include/hw/core/cpu.h
@@ -1090,6 +1090,12 @@ static inline void cpu_check_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
 {
 }
 
+static inline bool cpu_probe_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
+                                        int flags)
+{
+    return false;
+}
+
 static inline int cpu_watchpoint_address_matches(CPUState *cpu,
                                                  vaddr addr, vaddr len)
 {
@@ -1104,6 +1110,7 @@ void cpu_watchpoint_remove_by_ref(CPUState *cpu, CPUWatchpoint *watchpoint);
 void cpu_watchpoint_remove_all(CPUState *cpu, int mask);
 void cpu_check_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
                           MemTxAttrs attrs, int flags, uintptr_t ra);
+bool cpu_probe_watchpoint(CPUState *cpu, vaddr addr, vaddr len, int flags);
 int cpu_watchpoint_address_matches(CPUState *cpu, vaddr addr, vaddr len);
 #endif
 
diff --git a/exec.c b/exec.c
index 0cc500d53a..2b8f601b9e 100644
--- a/exec.c
+++ b/exec.c
@@ -2735,6 +2735,25 @@ void cpu_check_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
     }
 }
 
+bool cpu_probe_watchpoint(CPUState *cpu, vaddr addr, vaddr len, int flags)
+{
+    CPUClass *cc = CPU_GET_CLASS(cpu);
+    CPUWatchpoint *wp;
+
+    assert(tcg_enabled());
+
+    addr = cc->adjust_watchpoint_address(cpu, addr, len);
+    QTAILQ_FOREACH(wp, &cpu->watchpoints, entry) {
+        if (watchpoint_address_matches(wp, addr, len) && 
+            (wp->flags & flags) &&
+            (!(wp->flags & BP_CPU) ||
+             !cc->debug_check_watchpoint(cpu, wp))) {
+            return true;
+        }
+    }
+    return false;
+}
+
 static MemTxResult flatview_read(FlatView *fv, hwaddr addr,
                                  MemTxAttrs attrs, void *buf, hwaddr len);
 static MemTxResult flatview_write(FlatView *fv, hwaddr addr, MemTxAttrs attrs,
-- 
2.20.1

On Wed, 11 Mar 2020 at 06:44, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> Allow probing of a watchpoint *without* raising an exception.
> This is of most use for no-fault loads, which should indicate
> via some architectural means that the load did not occur.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  include/hw/core/cpu.h |  7 +++++++
>  exec.c                | 19 +++++++++++++++++++
>  2 files changed, 26 insertions(+)
>
> diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
> index 73e9a869a4..8ec44267a4 100644
> --- a/include/hw/core/cpu.h
> +++ b/include/hw/core/cpu.h
> @@ -1090,6 +1090,12 @@ static inline void cpu_check_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
>  {
>  }
>
> +static inline bool cpu_probe_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
> +                                        int flags)
> +{
> +    return false;
> +}
> +
>  static inline int cpu_watchpoint_address_matches(CPUState *cpu,
>                                                   vaddr addr, vaddr len)
>  {
> @@ -1104,6 +1110,7 @@ void cpu_watchpoint_remove_by_ref(CPUState *cpu, CPUWatchpoint *watchpoint);
>  void cpu_watchpoint_remove_all(CPUState *cpu, int mask);
>  void cpu_check_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
>                            MemTxAttrs attrs, int flags, uintptr_t ra);
> +bool cpu_probe_watchpoint(CPUState *cpu, vaddr addr, vaddr len, int flags);

Could we have a doc comment for the new function?

>  int cpu_watchpoint_address_matches(CPUState *cpu, vaddr addr, vaddr len);
>  #endif
>
> diff --git a/exec.c b/exec.c
> index 0cc500d53a..2b8f601b9e 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -2735,6 +2735,25 @@ void cpu_check_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
>      }
>  }
>
> +bool cpu_probe_watchpoint(CPUState *cpu, vaddr addr, vaddr len, int flags)
> +{
> +    CPUClass *cc = CPU_GET_CLASS(cpu);
> +    CPUWatchpoint *wp;
> +
> +    assert(tcg_enabled());
> +
> +    addr = cc->adjust_watchpoint_address(cpu, addr, len);
> +    QTAILQ_FOREACH(wp, &cpu->watchpoints, entry) {
> +        if (watchpoint_address_matches(wp, addr, len) &&
> +            (wp->flags & flags) &&
> +            (!(wp->flags & BP_CPU) ||
> +             !cc->debug_check_watchpoint(cpu, wp))) {
> +            return true;
> +        }
> +    }
> +    return false;
> +}

Clearly the insn emulation needs to do the right thing for
guest architectural watchpoints, but should a gdb watchpoint
also affect no-fault-load behaviour? I suppose making them
both behave the same way is probably the least-surprising choice.

thanks
-- PMM

On 4/16/20 5:08 AM, Peter Maydell wrote:
>>  void cpu_check_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
>>                            MemTxAttrs attrs, int flags, uintptr_t ra);
>> +bool cpu_probe_watchpoint(CPUState *cpu, vaddr addr, vaddr len, int flags);
> 
> Could we have a doc comment for the new function?
> 
>>  int cpu_watchpoint_address_matches(CPUState *cpu, vaddr addr, vaddr len);

Hah.  In the process of doing that, I notice that
cpu_watchpoint_address_matches actually does what I want.

I have added documentation for cpu_check_watchpoint and
cpu_watchpoint_address_matches and have dropped this new function.

> Clearly the insn emulation needs to do the right thing for
> guest architectural watchpoints, but should a gdb watchpoint
> also affect no-fault-load behaviour? I suppose making them
> both behave the same way is probably the least-surprising choice.

In both cases we need to interrupt the execution in order to actually honor the
watchpoint.  So yes, treating them the same seems the only reasonable way.


r~