Re: Register fudging (CRISv32)

[Daniel Jacobowitz <drow at false dot org>]

On Fri, Sep 03, 2004 at 02:37:03PM +0200, Orjan Friberg wrote:
> My upcoming CRISv32 port (remote target, Linux based) is starting to 
> look pretty good(*) but I'm left with a nagging feeling that the 
> register fudging I'm doing isn't necessarily done where it should be 
> and/or the right way.  Right now it's being done in three different 
> places (this relating to debugging user-mode programs):
> 
> (1) in the kernel
> (2) in the Gdbserver
> (3) in GDB
> 
> Basically, what I would like to hear is people's opinions on how various 
> kinds of register fudging should be done.
> 
> On to the details:
> 
> * The first fudging is the equivalent to DECR_PC_AFTER_BREAK, though 
> it's not using that mechanism in GDB; instead it's being done in the 
> kernel.  On one hand I feel more comfortable doing it in the kernel 
> where I know exactly what happens; on the other hand the decrementation 
> needs to be duplicated in, for example, a classic kernel gdb stub. 
> Should I be using DECR_PC_AFTER_BREAK in GDB instead?  Or the 
> implementation in the Gdbserver?

Up to you.  I think doing it in the kernel stub and kernel ptrace
support is a better strategy, esp. if you have additional information
confirming that a breakpoint was hit.

> * Another fudging that takes place is the filling in of a pseudo-PC 
> register (there is no actual PC register, so it's not present in struct 
> pt_regs).  This is being done in the Gdbserver.  In addition, in case we 
> stopped in a delay slot, I *may* need to look at the code to determine 
> what the PC should be set to (meaning I can't rely on register contents 
> alone).  I've found 3 cases where this needs to be done:
> 
>   (1) In case of a stop (break, h/w watchpoint, receiving a signal etc)
>   (2) When unwinding a sigtramp frame
>   (3) When loading a core dump (supply_gregset)
> 
> As of now, delay-slot-adjustment of the PC is only being done for the 
> first case (normal stop), and it's also done in the Gdbserver.  The 
> other two cases don't handle being stopped in a delay slot yet, though I 
> have a hunch this could be done in GDB.

There's arguments both ways for this.  For instance, I think it would
be reasonable to do this in the kernel.

> * In addition to this, I need to set the h/w single-step PC to 0 in the 
> kernel at various times, but I've seen other architectures doing that 
> and I feel pretty confident that is the right way to do it.

Not sure what you mean by this.

-- 
Daniel Jacobowitz