Saving to 32 bits of GPRs in signal context

[Kumar Gala]

On May 29, 2007, at 10:37 PM, Paul Mackerras wrote:

> 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).

I like this, it means we can error if HW doesn't support it and  
requires applications to do something specific to enable the feature.

- k