Re: [Qemu-devel] [PATCH] Make cpu_single_env thread local (Linux only for now)

[Peter Maydell]

On 26 October 2011 16:02, Jan Kiszka <address@hidden> wrote:
> On 2011-10-26 16:54, Peter Maydell wrote:
>> On 26 October 2011 15:39, Paolo Bonzini <address@hidden> wrote:
>>> On 10/26/2011 04:03 PM, Peter Maydell wrote:
>>>>
>>>> For the record (since I think we only talked about this on IRC):
>>>>  * the POSIX TLS fallback code doesn't work on Linux hosts for
>>>>    linux-user emulation (the constructor is never called to set up
>>>>    the TLS for the main thread, probably something to do with our
>>>>    custom linker script, since it does work OK for system emulation)
>>>>  * if I recall correctly from IRC it doesn't compile on OpenBSD
>>>>
>>>> so for 1.0 perhaps we need to fall back to a simpler set of patches
>>>> that just avoid the regression in thread support for linux-user ?
>>>
>>> I agree.
>>
>> I was thinking something like a trivial qemu-tls.h which does
>>
>> #ifdef __linux__
>> #define DECLARE_TLS(type, x) extern DEFINE_TLS(type, x)
>> #define DEFINE_TLS(type, x)  __thread __typeof__(type) tls__##x
>> #define get_tls(x)           tls__##x
>> #else
>> /* Dummy implementations -- we can get away with this because system
>>  * mode is effectively single-threaded for our current limited use of
>>  * TLS, and the only -user mode which supports multiple threads is
>>  * linux-user.
>
> We will use it in system mode as well, e.g. when I'll push away the
> global lock from KVM entry/exits. So let's state that it is only
> interesting on Linux hosts for now.
>
> Otherwise, this looks good.

The point of the comment is that you can't use this facility in
system mode until we've added the non-Linux support (because you'd
break Win32 &c). So fixing and committing the non-Linux support is
a prerequisite for anything like the global lock moves. I can be
a bit more explicit about that if you like:

/* Dummy implementations -- we can get away with this because system
 * mode is effectively single-threaded for our current limited use of
 * TLS, and the only -user mode which supports multiple threads is
 * linux-user.
 * This means you cannot use this for any variables which will
 * actually be accessed by more than one thread in system mode
 * until the implementations for Win32 and POSIX systems without
 * __thread have been added!
 *
 * TODO: add implementations via Win32 .tls sections and
 * POSIX pthread_getspecific.
 */

-- PMM