Re: [Qemu-devel] [PATCH] configure: Detect when glibc implements makecontext() to always fail

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/14/2011 02:30 PM, Peter Maydell wrote:
> I've also just discovered that it's no use on Oneiric,
> where the linker warning has gone away but the syscall
> still always returns ENOSYS.
>
> I think we should just always use the gthread implementation
> rather than preferring a non-portable-and-hard-to-detect
> set of functions (which increases the set of different
> configs we need to test with). If there's a performance problem
> with that we should get it fixed in gthread:-)

A user-space longjmp will always be slower than a mutex+condvar+context 
switch.  We're talking _orders of magnitude_ slower.  At this point it's 
better to write assembly, since we already support only a dozen TCG targets.

I played with an alternative implementation using a 2-barrier instead of 
mutex+condvar, but it didn't give any speedup and was still much slower 
than gthread.

Paolo