Re: Using hardware watchpoint for applications debugging

[Nigel Stephens <nigel@mips.com>]

colin wrote:
> As to adding watchpoint to kernel, there will be another problem.
> ASID in kernel is variable. Therefore, we cannot indicate which thread we
> want to watch by ASID.
> What we can do is setting G (global) bit to WatchHi Register and then all
> threads accessing that address will cause the exception.
> In the exception, it will filter the threads by PID to find out the thread
> we are watching.
>
>   

They're variable, but not very variable: the PID->ASID mapping will only
change when the ASIDs roll over and the ASID gets reallocated to a
different process, which will only happen after another 256 processes
have been created. But in that case your watched process will have to be
allocated a new ASID before it can run again. So you could, perhaps,
modify the TLB management code to clear the Watch registers whenever an
ASID belong to a process with watchpoints is recycled, and then
reprogram the Watch registers when such a process is allocated a new
ASID. Alternatively you could maintain pre-process copies of the Watch
registers, and context switch them along with other per-process register
state -- though that is adding context switch overhead to processes
which don't use watchpoints, and might not be popular with the maintainer.

Nigel