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

From: Sergey Fedorov
Subject: Re: [Qemu-devel] [RFC 0/7] Safe watch and breakpoint manipulation
Date: Mon, 20 Jun 2016 21:19:20 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0

On 20/06/16 19:08, Paolo Bonzini wrote:
>>> 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.

It's a good idea to use tb_lock for breakpoints. Actually, we could use
spinlocks for watchpoints, if we're careful. But I don't this it can be
so critical in performance.

Kind regards,

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