Re: [RFA][branch] Fix DVC calculation for booke ppc

["Ulrich Weigand" <uweigand@de.ibm.com>]

Thiago Jung Bauermann wrotE:

> The bug was introduced by a patch of mine (doh).
> ppc-linux-nat.c:calculate_dvc uses the watchpoint length that is passed
> to target_insert_watchpoint to calculate the contents of the Data Value
> Compare register. The problem is that for the ranged watchpoints feature
> I changed GDB to pass 1 as length if the "set powerpc exact-watchpoints"
> flag is on. This messes up things for calculate_dvc.
> 
> This patch makes check_condition obtain the length of the watchpoint
> region from the condition expression, so that it can be passed to
> calculate_dvc. This works because for a condition to be eligible for
> hardware acceleration, it needs to have a strict form:
> 
> (gdb) watch ADDRESS|VARIABLE \
>        if  ADDRESS|VARIABLE == CONSTANT EXPRESSION
> 
> which means that the ADDRESS|VARIABLE part is the same as the watchpoint
> region.

Huh.  That strikes me as a hack to work around another hack :-/

I may have missed some of the discussion behind the current implementation.
Could you explain again:

- why you're passing an incorrect length of 1 if the "exact watchpoints"
  flag is on?

- why you're only supporting hardware-accelerated conditions if the length
  is 1?

Note that while you say that the condition needs to have a strict form,
you don't currently actually *verify* this: if you have a command of the
form "watch A if B == C", you only verify that A and B *start* at the
same address -- you really need to also verify that A and B have the
same length -- but you cannot because the length of A is not available
to the target since you're always getting 1 for length.


It seems to me the "right way" would be for the common parts to always
pass correct information (address, length, condition, ...) to the target,
and then for the target to look at the request and choose the best
possible hardware means to implement this particular request ...

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU Toolchain for Linux on System z and Cell BE
  Ulrich.Weigand@de.ibm.com