Messages in this thread | | | Date | Wed, 11 Mar 2009 14:35:33 +0100 | From | Ingo Molnar <> | Subject | Re: [patch 01/11] Introducing generic hardware breakpoint handler interfaces |
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* K.Prasad <prasad@linux.vnet.ibm.com> wrote:
> For the benefit of continuing discussion on this topic, here's > an extract from an old mail > (http://lkml.org/lkml/2007/2/5/465) from Roland, explaining > the need for prioritisation of requests. It must have been > utrace as a potential user that made him suggest this. > > "I am all in favor of a facility to manage shared use of the > debug registers, such as your debugreg.h additions. I just > think it needs to be a little more flexible. An unobtrusive > kernel facility has to get out of the way when user-mode > decides to use all its debug registers. It's not immediately > important what it's going to about it when contention arises, > but there has to be a way for the user-mode facilities to say > they need to allocate debugregs with priority and evict other > squatters. So, something like code allocating a debugreg can > supply a callback that's made when its allocation has to taken > by something with higher priority. > > Even after utrace, there will always be the possibility of a > traditional uncoordinated user of the raw debug registers, if > nothing else ptrace compatibility will always be there for old > users. So anything new and fancy needs to be prepared to back > out of the way gracefully. In the case of kwatch, it can just > have a handler function given by the caller to start with. > It's OK if individual callers can specially declare "I am not > well-behaved" and eat debugregs so that well-behaved > high-priority users like ptrace just have to lose (breaking > compatibility). But no well-behaved caller of kwatch will do > that. > > I certainly intend for later features based on utrace to > include higher-level treatment of watchpoints so that user > debugging facilities can also become responsive to debugreg > allocation pressure. (Eventually, the user facilities might > have easier ways of falling back to other methods and getting > out of the way of kernel debugreg consumers, than can be done > for the kernel-mode-tracing facilities.) To that end, I'd > like to see a clear and robust interface for debugreg sharing, > below the level of kwatch."
This argument ignores the reality of debug registers: overcommitted usage of them causes silent failures and unobvious behavior.
I think the simple reservation scheme i outlined in the previous mail is the minimum amount of complexity that still gets kernel-space hw-breakpoints going robustly. If we add anything more fancy we want it based on actual need and desire.
Ingo
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