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Date: Fri, 28 Jun 2013 17:11:19 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [Qemu-trivial] [PATCH] tls.h: Enable TLS on FreeBSD
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-trivial@nongnu.org, Ed Maste <emaste@freebsd.org>, Michael Tokarev <mjt@tls.msk.ru>, qemu-devel <qemu-devel@nongnu.org>

Il 28/06/2013 16:55, Peter Maydell ha scritto:
>>> >> I don't understand what you mean by "no need for it". We are
>>> >> already multithreaded in system mode, so this is simply
>>> >> making FreeBSD do the same thing as Linux.
>> >
>> > cpu_single_env is protected by the BQL unless you're running on KVM.
> That doesn't make it magically not a per-thread variable.

If it were possible to say "make cpu_single_env magically not per-thread
on Linux/TCG", I'd be all for it!

But I _can_ make cpu_single_env not per-thread on FreeBSD/TCG, because
no one needs per-thread cpu_single_env on FreeBSD.

If we could drop support for platforms that lack __thread (or some other
fast TLS mechanism), then it would be another story of course.  We would
not have to do things like this for qtest:

    while (1) {
        cpu_single_env = NULL;
        qemu_mutex_unlock_iothread();
        do {
            int sig;
            r = sigwait(&waitset, &sig);
        } while (r == -1 && (errno == EAGAIN || errno == EINTR));
        if (r == -1) {
            perror("sigwait");
            exit(1);
        }
        qemu_mutex_lock_iothread();
        cpu_single_env = env;
        qemu_wait_io_event_common(cpu);
    }

or similarly set cpu_single_env in cpu_exec.

And of course if bsd-user supported 1:1 mapping between guest and host
threads on FreeBSD, cpu_single_env would have to be thread-local.

Paolo