On Wed, 27 Jun 2007, Roland McGrath wrote:
> In the first battle just to make it compile, the only issue was that you
> assume every machine has TIF_DEBUG, which is in fact an implementation
> detail chosen lately by i386 and x86_64. AFAIK the only reason for it
> there is just to make a cheap test of multiple bits in the hot path
> deciding to call __switch_to_xtra. Do you rely on it meaning something
> more precise than just being a shorthand for hw_breakpoint_info!=NULL?
Going over the code, I remembered that TIF_DEBUG really does mean moree
than just hw_breakpoint_info != NULL. It means that the thread
actually has some breakpoints registered.
Why keep the hw_breakpoint_info structure if there are no registered
breakpoints? I did it so that the virtualized DR[0-3] values would
remain intact.
For other processors that have only one debug register, this won't
matter so much. But of course there are references to TIF_DEBUG in the
arch-independent code. Do you think there would be any problem about
reserving a bit for TIF_DEBUG in the other architectures?
Alan Stern
P.S.: I'm just now getting around to doing the stuff we discussed last
week. It has been a busy time... At OLS somebody asked when
hw-breakpoint would get into the mainline. I guessed that it would be
a few months before it is added to -mm. Solving this
pre/post-notification issue will be difficult.
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