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Re: [Qemu-devel] Rationalising exit_request, cpu->exit_request and tcg_e
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] Rationalising exit_request, cpu->exit_request and tcg_exit_req? |
Date: |
Wed, 16 Dec 2015 18:21:48 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0 |
On 16/12/2015 18:14, Alex Bennée wrote:
>
> While looking at Fred's current MTTCG WIP branch I ran into a problem
> where:
>
> - async_safe_work_pending() was true
> - this triggered setting cpu->exit_request
> - however we never left tcg_exec_all()
> - because the global exit_request wasn't set
> - hence qemu_tcg_wait_io_event() never drained the async work queue
exit_request should disappear with MTTCG. It should only have
cpu->tcg_exit_req and cpu->exit_request.
> While trying to understand why we have both a cpu and a global
> exit_request I then discovered there is also cpu->tcg_exit_req which is
> the actual variable the TCG examines. This leads to sequences like:
> This seems to me to be slightly insane as we now have 3 variables that
> struggle to be kept in sync. Could all this not be rationalised into a
> single variable or am I missing a subtly in their different semantics?
They do.
cpu->tcg_exit_req is the one that is set from generated code. It is set
if you do not have to exit cpu_exec.
cpu->exit_request and exit_request are both necessary, in order to
synchronize exits with the setting of tcg_current_cpu. Again, this is
only needed in single-threaded TCG, because MTTCG gets rid of
tcg_current_cpu. It's documented here:
http://article.gmane.org/gmane.comp.emulators.qemu/357210.
> I don't know if there is clean-up that can happen in master or if this
> all needs to be done in the mttcg work but would it make sense just to
> keep cpu->exit_request,
Yes, and it's actually necessary. :)
> I did have a brief look at the KVM side of the code and it only seems to
> reference cpu->exit_request so I think the rest of this is a TCG
> problem.
Yes. With MTTCG you still need cpu->tcg_exit_req because you still have
something like KVM's kernel- and userspace-vmexits. In TCG the
lightweight exits are those that do not leave cpu_exec. Those only set
cpu->tcg_exit_req.
Paolo