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SubjectRe: [patch 01/11] Introducing generic hardware breakpoint handler interfaces
On Tue, 10 Mar 2009, Ingo Molnar wrote:

> * prasad@linux.vnet.ibm.com <prasad@linux.vnet.ibm.com> wrote:
>
>
> > +static u8 tprio[HB_NUM]; /* Thread bp max priorities */
> > +LIST_HEAD(kernel_bps); /* Kernel breakpoint list */
> > +static LIST_HEAD(thread_list); /* thread_hw_breakpoint list */
> > +DEFINE_PER_CPU(struct cpu_hw_breakpoint, cpu_bp);

If nobody minds, I'll answer some of these questions on Prasad's behalf
because they address parts of the code that were written before he took
over the project.

> hm, why do we need the whole 'priority' mechanism? It seems very
> over-designed to me.

This was done at Roland McGrath's express request. We should see what
he has to say about it.

> The likelyhood of both user-space and kernel-space to use
> hw-breakpoints is very low to begin with. And if they use them,
> the likelyhood of there being more than 4 debugregs required in
> the same context is even lower.

Not all architectures have 4 debug registers. Most have only one.

> If that happens we shouldnt try to be too smart about them -
> just override user-space ones with kernel space ones and that's
> it. No explicit priorities are needed.

Roland really did not want it done this way.

Alan Stern



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