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Date: Thu, 15 Jan 2015 12:12:54 +0100
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Subject: Re: [Qemu-devel] global_mutex and multithread.
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To: Frederic Konrad <fred.konrad@greensocs.com>, mttcg@listserver.greensocs.com, qemu-devel <qemu-devel@nongnu.org>
Cc: Peter Maydell <peter.maydell@linaro.org>, Alexander Graf <agraf@suse.de>

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On 15/01/2015 11:25, Frederic Konrad wrote:
> Hi everybody,
> 
> In case of multithread TCG what is the best way to handle
> qemu_global_mutex?
> We though to have one mutex per vcpu and then synchronize vcpu threads when
> they exit (eg: in tcg_exec_all).
> 
> Is that making sense?

The basic ideas from Jan's patch in
http://article.gmane.org/gmane.comp.emulators.qemu/118807 still apply.

RAM block reordering doesn't exist anymore, having been replaced with
mru_block.

The patch reacquired the lock when entering MMIO or PIO emulation.
That's enough while there is only one VCPU thread.

Once you have >1 VCPU thread you'll need the RCU work that I am slowly
polishing and sending out.  That's because one device can change the
memory map, and that will cause a tlb_flush for all CPUs in tcg_commit,
and that's not thread-safe.

And later on, once devices start being converted to run outside the BQL,
that can be changed to use new functions address_space_rw_unlocked /
io_mem_read_unlocked / io_mem_write_unlocked.  Something like that is
already visible at https://github.com/bonzini/qemu/commits/rcu (ignore
patches after "kvm: Switch to unlocked MMIO").

Paolo