Saving to 32 bits of GPRs in signal context

[Paul Mackerras]

Steve Munroe writes:

> But unless you take the time to write it up like a full ABI change you are
> never sure that it IS compatible. And any change to the size/shape of
> ucontext_t is an ABI change.

There is no change to the size or shape of the ucontext_t.  There is
no change to the ABI at all, in the sense that everything that is
currently guaranteed by the ABI is still guaranteed.  An extra
guarantee is added: the top 32 bits of the GPRs will not change
unpredictably as long as you don't call a function and don't use
setcontext or swapcontext to return from a signal handler.

I think actually it would be useful to have the saving/restoring of
the high 32 bits controlled by a prctl, so that programs have to ask
explicitly for the new behaviour (and programs that don't want to use
the high 32 bits don't incur the extra overhead).

Paul.