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Re: [Qemu-devel] [PATCH] tls.h: Enable TLS on FreeBSD

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH] tls.h: Enable TLS on FreeBSD
Date: Tue, 25 Jun 2013 08:18:33 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6

Il 24/06/2013 23:30, Peter Maydell ha scritto:
> On 24 June 2013 22:15, Paolo Bonzini <address@hidden> wrote:
>> Il 24/06/2013 21:21, Ed Maste ha scritto:
>>> Signed-off-by: Ed Maste <address@hidden>
>>> ---
>>> I have had this in a local tree for some time, and it is needed by the
>>> BSD-user work that is now being proposed.
>> At this time, qemu/tls.h is really just for cpu_single_env, so I think
>> this patch should be applied together with the bsd-user patches that
>> need it.
> This patch has arrived because I suggested splitting it out from
> those ;-) System mode qemu is multi-threaded, so moving more host
> OSes into the fold of "these variables behave like the linux host
> which is where the bulk of testing happens" sounds like a good idea
> to me.

Ah. :)

>>> As an aside, an abstraction was recently proposed for Open vSwtich that
>>> can use any of _Thread_local, __thread, or pthread_getspecific() which
>>> may make a convenient reference for someone wishing to implement one of
>>> the TODOs: http://openvswitch.org/pipermail/dev/2013-June/028665.html
>> I and Stefan Hajnoczi have almost the same idea implemented in QEMU
>> (except that get_foo() returns a pointer to the variable).  But
>> pthread_get/setspecific would be too slow for cpu_single_env, so we're
>> just switching to __thread for cpu_single_env (for Linux in our patches,
>> but you can add FreeBSD too once it's needed).
> I would really like cpu_single_env (and the other thread variables)
> to have consistent semantics everywhere (which means "always thread
> local"), even if that means "sometimes suboptimal performance".
> Reasoning about a variable that might or might not be per-thread
> is just painful. Alternatively, we could just drop support for any
> host OS that doesn't provide reasonably efficient per-thread data.

That is basically only OpenBSD.  If we get reasonable support for
FreeBSD, I'm perfectly fine with dropping OpenBSD.  But so far OpenBSD
was the only officially supported variant.


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