[v4] powerpc: Export thread_struct.used_vr/used_vsr to user space

[Michael Ellerman]

Laurent Dufour <ldufour at linux.vnet.ibm.com> writes:
> On 07/07/2016 15:21, Benjamin Herrenschmidt wrote:
>> On Thu, 2016-07-07 at 15:12 +0200, Laurent Dufour wrote:
>>> Most of the time this is fine, but in the case a thread which has really
>>> used those registers is catching a signal just after the restore and
>>> before it has touched to these registers again (and so set used_vsr/vr),
>>> these registers will not be pushed in the newly built signal frame since
>>> setup_sigcontext() check for used_vsr/vr before pushing the registers on
>>> the stack.
>>> This may be an issue in the case the thread wants to changed those
>>> registers (don't ask me why :)) in the stacked signal frame from the
>>> signal handler since they will not be there...
>>>
>>> Being able to get and set the used_vr and used_vsr thread's variables,
>>> fixes this issue.
>> 
>> I think the right fix is that if a restore_sigcontext() has the MSR bits set,
>> it should set the corresponding used_* flag.
>> 
>> Or is there a reason why that won't work ?
>
> I got your point and I agree that most of the time now, the Altivec/VSX
> registers are used by libc. In that case is there still a need for the
> lazy Altivec/VSX registers dump in the signal frame ?

Probably not for new programs. But it could conceivably break old
software.

> I'm fine with your proposal, except that every restarted process will
> have the used_vr/used_vsx turned on after the restart since we can't
> check if these registers were used or not at checkpoint time.
> But that may be a minor point...

Yeah I'd argue that's not worth worrying about, at least for now.

If it *is* a problem then we can fix it later.

cheers