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Re: [Qemu-devel] [RFC v3] translate-all: protect code_gen_buffer with RC

From: Emilio G. Cota
Subject: Re: [Qemu-devel] [RFC v3] translate-all: protect code_gen_buffer with RCU
Date: Fri, 29 Apr 2016 23:40:35 -0400
User-agent: Mutt/1.5.23 (2014-03-12)

On Tue, Apr 26, 2016 at 07:32:39 +0100, Alex Bennée wrote:
> Emilio G. Cota <address@hidden> writes:
> > With two code_gen "halves", if two tb_flush calls are done in the same
> > RCU read critical section, we're screwed. I added a cpu_exit at the end
> > of tb_flush to try to mitigate this, but I haven't audited all the callers
> > (for instance, what does the gdbstub do?).
> I'm not sure we are going to get much from this approach. The tb_flush
> is a fairly rare occurrence its not like its on the critical performance
> path (although of course pathological cases are possible).

This is what I thought from the beginning, but wanted to give this
alternative a go anyway to see if it was feasible.

On my end I won't do any more work on this approach. Will go back
to locks, despite Paolo's (justified) dislike for them =)

> > If we end up having a mechanism to "stop all  CPUs to do something", as
> > I think we'll end up needing for correct LL/SC emulation, we'll probably
> > be better off using that mechanism for tb_flush as well -- plus, we'll avoid
> > wasting memory.
> I'm fairly certain there will need to be a "stop everything" mode for
> some things - I'm less certain of the best way of doing it. Did you get
> a chance to look at my version of the async_safe_work mechanism?

Not yet, but will get to it very soon.



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