Re: [Qemu-devel] [RFC 0/7] Safe watch and breakpoint manipulation

["Alex Bennée" <alex.bennee@linaro.org>]

Paolo Bonzini <pbonzini@redhat.com> writes:

>> > The patch series changes things in stages.
>> >
>> > First we move the break/watchpoints into an array which is more
>> > amenable to RCU control that the QLIST. We then control the life time
>> > of references to break/watchpoint data by removing long held
>> > references in the target code and getting information when needed from
>> > the core. Then we stop dynamically allocation the watch/breakpoint
>> > data and store it directly in the array which makes iteration across
>> > the list a bit more cache friendly than referenced pointers. Finally
>> > addition and removal of elements of the array is put under RCU
>> > control. This ensures there is always a safe array of data to check
>> > in the run-loop.
>>
>> I a little bit unsure if we really want to complicate things with RCU.
>> Why don't we simply protect the lists with a mutex given that there's no
>> contention expected? BTW, as it comes to debugging, I suppose we don't
>> expect great performance anyway.
>
> Mutexes do introduce some overhead.  The breakpoints list are mostly touched
> during translation, but watchpoints aren't so we could use tb_lock for
> breakpoints and a separate per-CPU mutex for watchpoints.  That could
> indeed work.

The watchpoint contention is the biggest one. FWIW I like the RCU
approach because it is low impact when running (and I'm hoping faster as
well by not being a linked list).

It's not a major problem in system mode because generally the system is
halted when changes are made to the list. However I'd like to solve it
properly for both system and user-mode so I can then forgot about
another special case.

--
Alex Bennée