Re: [patch v8 17/24] record-btrace: provide xfer_partial target method

[Pedro Alves <palves at redhat dot com>]

On 12/17/2013 11:57 AM, Metzger, Markus T wrote:
>> -----Original Message-----
>> From: Pedro Alves [mailto:palves@redhat.com]
>> Sent: Monday, December 16, 2013 8:16 PM
> 
> 
>>> I changed the return -1 to throw_error (...) and added a check for
>>> writebuf != NULL.  Suddenly I got tons of errors when GDB can't insert
>>> breakpoints any longer for (reverse-)stepping.
>>
>> This is why precord keeps track of breakpoints itself too:
> [...] 
>>>   Also stepping gets broken.
>>
>> I can't immediately why that would be.
> 
> Because we can't set temporary breakpoints.
> 
> 
>>> I now get an error when trying to access a variable with static storage
>>> duration or when trying to access memory directly via its address.
>>> It would be nice to also get an <unavailable> in those cases.  This would
>>> require the respective layer to catch my exception.
>>
>> Please try returning TARGET_XFER_E_UNAVAILABLE instead.
> 
> That is ignored just like the -1 I returned earlier.  I nevertheless changed
> the default return to that since it is more descriptive.

Thanks.  Hmm, yes, looks like raw_memory_xfer_partial carries on
looking at the target beneath, and then when that fails, we'll
lose TARGET_XFER_E_UNAVAILABLE, and return TARGET_XFER_E_IO/-1,
losing the better TARGET_XFER_E_UNAVAILABLE.

>>> To avoid those errors when trying to set breakpoints, I could try
>>> providing to_insert_breakpoint and to_remove_breakpoint methods
>>> and maintain my own breakpoints.
>>
>> Right.
> 
> I have something to temporarily disable the xfer checks during
> to_insert_breakpoint and to_remove_breakpoint.
> 
> Not sure whether this is considered too hacky or what else I'm missing.

It's hacky as the breakpoints in memory will never actually
trigger/execute.  If you want to assume that the inferior's current
read only sections match exactly the read only sections the program
had when the trace was taken, I won't insist.  The assumption will
fail across tracing e.g., dlopen/dlclose/mmap/unmmap, as breakpoints
will fail to insert on unmapped sections.

> My tests all pass.  Any idea where else GDB would need to access
> target memory in order to function correctly?

Can't think of anything.

> Here's the patch.  I omit a preparation patch to pass target_ops to
> to_insert_breakpoint and to_remove_breakpoint so that the request
> can be forwarded to the target beneath.
> 
> diff --git a/gdb/record-btrace.c b/gdb/record-btrace.c
> index 00a056d..0536071 100644
> --- a/gdb/record-btrace.c
> +++ b/gdb/record-btrace.c
> @@ -42,6 +42,9 @@ static struct target_ops record_btrace_ops;
>  /* A new thread observer enabling branch tracing for the new thread.  */
>  static struct observer *record_btrace_thread_observer;
>  
> +/* Temporarily allow memory accesses.  */
> +static int record_btrace_allow_memory_access;
> +
>  /* Print a record-btrace debug message.  Use do ... while (0) to avoid
>     ambiguities when used in if statements.  */
>  
> @@ -805,7 +808,7 @@ record_btrace_xfer_partial (struct target_ops *ops, enum target_object object,
>    struct target_ops *t;
>  
>    /* Filter out requests that don't make sense during replay.  */
> -  if (record_btrace_is_replaying ())
> +  if (record_btrace_allow_memory_access == 0 && record_btrace_is_replaying ())

We use ! for boolean ints, so write:

  if (!record_btrace_allow_memory_access && record_btrace_is_replaying ())

-- 
Pedro Alves