Re: Is stub support for the 's' packet optional or required?

[Andrew Cagney <ac131313@redhat.com>]

> On Tue, Feb 18, 2003 at 09:29:58AM -0700, Kevin Buettner wrote:
> 
>> On Feb 17,  9:04pm, Andrew Cagney wrote:
>> 
> 
>> > If GDB implements software single step, then the `s' packet is never
>> > used.  Consequently, requiring the unconditional implementation of "s"
>> > makes little sense.

Kevin wrote:

>> What about the situation where GDB implements software single step AND
>> the stub implements the 's' packet?  Shouldn't GDB at least attempt to
>> see if the stub supports the 's' packet before deciding to never send
>> it?

It should but the interaction is weird.  remote.c doesn't see the "" 
reply until target_wait() is called.  This means that the target_wait() 
method would need to be modified to handle this.  I guess it could 
record this and then return immedatly with a TARGET_WAITKIND_SPURIOUS. 
Kind of vaguely like how some of the other packets are handled.

But note, I'm guessing.  Just having commands to disable it would be a 
good first draft.

Oh, and yes.  I really have seen targets that neither had h/w single 
step nor had the space to implement s/w single step locally.

> In my humble opinion, SOFTWARE_SINGLE_STEP should affect native code
> and not remote;

No.  The relevant comments read:

# FIXME/cagney/2001-01-18: This should be split in two.  A target method 
that indicates if the target needs software single step.  An ISA method 
to implement it.
#
# FIXME/cagney/2001-01-18: This should be replaced with something that 
inserts breakpoints using the breakpoint system instead of blatting 
memory directly (as with rs6000).
#
# FIXME/cagney/2001-01-18: The logic is backwards.  It should be asking 
if the target can single step.  If not, then implement single step using 
breakpoints.

(All taken with a grain of salt.)

So, from the point of view of GDB's architecture, there is no difference.

 > I'm much too intimidated by the stop and resume logic
 > to actually change it myself, though.  If there were less global state
 > around infrun this might be easier.

Sigh.

>> [For remote MIPS/Linux targets, I've found some cases where GDB's
>> implementation of software singlestep causes some undesirable behavior
>> when doing the 'stepi' operation through some code that's hit by a number
>> of threads.  Yet, when software single step is implemented in the debug
>> agent (and disabled in GDB), the debugging behavior is much more useful
>> (and sensible).]
> 
> 
> Is it just slow, or do different things actually happen?

It is just very slow.

Andrew