RFC: Unpredictable register set operations

[Kevin Buettner]

On Jul 15,  6:09pm, Daniel Jacobowitz wrote:

> Consider PowerPC and the $ps register (MSR).  When debugging a kernel or
> embedded application, GDB has pretty complete control (?) over this
> register.  In GNU/Linux userspace, however, only two bits of it can be set. 
> The rest are read-only.
> 
> So what happens if you "set $ps = 0"?  Well, the right thing happens, but
> until the next time the target stops "print $ps" will print 0.  Which is not
> actually the value of the $ps register.
> 
> Here's the options that I see:
>   - Ignore and document this.
>   - Refetch registers after storing them.
>   - Invalidate registers for lazy re-fetch after storing them.
>   - Add a target hook for might-be-volatile registers, and invalidate
>     only those registers after storing them - or don't cache them at
>     all.

One of the refetch options (lazy or not) sounds good to me.  The
target hook idea sounds like overkill, and I think we can do better
than "ignore and document".

Kevin