qemu on ARM hosts aborts on startup because makecontext() always fails

I have temporarily added a patch to qemu-linaro ("configure: Force ucontext check to fail on ARM") to work around this in the configure script. However this is only a stopgap to let us get on with KVM development and won't be going upstream.

Attached is a 1st cut of a makecontext/setcontext/getcontext/swapcontext set for ARM - not ready to go into libc yet;
currently it builds standalone and links (and passes) with a test of mine. Next stop libc and a lot of cleanup.

On Wed, Dec 21, 2011 at 5:42 PM, Dr. David Alan Gilbert
<email address hidden> wrote:
> Attached is a 1st cut of a makecontext/setcontext/getcontext/swapcontext set for ARM - not ready to go into libc yet;
> currently it builds standalone and links (and passes) with a test of mine.  Next stop libc and a lot of cleanup.

Excellent. There is a stand-alone QEMU coroutine test which you can
build with "make test-coroutine".

Stefan

OK, here is a eglibc patch that adds the context routines; seems to pass Stefan's magic coroutine test, pass all the context specific tests in eglibc and boots a debian image on qemu. (Not run a full eglibc test run yet).

Dave

Removing Dave as he's done most of the work but can't complete it.

Consider putting this in 2.15 as well. Talk with Matthias re: getting it into Ubuntu Precise either through a 2.13 backport or as part of a 2.15 update.

Plan:
(a) get makecontext added to eglibc
(b) see if upstream eglibc are open to the idea of some sort of #define for MAKECONTEXT_ACTUALLY_IMPLEMENTED_NOW so we can detect it at compile time
(c) otherwise, work around in qemu (probably by adding another layer of indirection)

David Gilbert's post to upstream eglibc, and a reply from Joseph Myers with some review comments:
http://comments.gmane.org/gmane.comp.lib.glibc.ports/1187

Michael Hope pointed out that glibc defines a macro __stub_makecontext when the makecontext() implementation is a stub, so we can just add this to the qemu configure test to make it automatically fall back to the pthreads version as required.

http://comments.gmane.org/gmane.comp.lib.glibc.ports/1325

-- Michael's version of the makecontext patches has been committed to glibc-ports and should be available in glibc 2.16.

Upstream qemu (and thus qemu-linaro) now have a version of the configure check which tests for __stub_makecontext, so qemu will just automatically pick up the makecontext implementation when compiled against a glibc which supports it. So I think the only remaining thing is to decide whether Linaro and/or Ubuntu want to backport this.

...and I think that backporting would be a bug against some other package, not qemu, so we can close this bug.

Fix released in upstream QEMU 1.1.