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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 18:49:20 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0

On 17/06/16 19:33, Alex Bennée wrote:
> Last time I went through the MTTCG code the access to the
> break/watchpoint code was annotated with "RCU?". The code currently
> gets away with avoiding locks for the gdbstub as the guest execution
> state is usually halted. However when used for modelling architectural
> debug registers there is no such protection.

I'm not so sure if there's any architecture which permits changing
breakpoins/watchpoints of one core from another.

> 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.

Kind regards,

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