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

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2016-07-07 at 15:12 +0200, Laurent Dufour wrote:
> 
> Basically, CRIU checkpoints the process register's state through the
> ptrace API, and it restores it through a signal frame at restart time.
> This is quite odd but that the way it works on all the CRIU's supported
> architectures.
> 
> Obviously everything is done from/in user space, so the sigframe
> building too.
> Since we can't know from user space if the thread has used or not the
> Altivec/VSX registers, since we can't rely on the MSR bits, we always
> dump these registers.

Right, however is that an issue ? These days with glibc using V{M,S}X
for things like memcpy I would think there is little to gain in trying
to avoid dumping them.

> > Alternately, when restoring, can you setup the sigframe with the Altivec/VSX
> > fields populated, and the kernel will then load them, regardless of whether
> > they were actually used or not prior to the checkpoint?
> 
> In the case of Altivec/VSX fields, we currently force the kernel to
> retrieve them from the signal frame by setting MSR_VEC/MSR_VSX so
> restore_sigcontext() will copy them to the kernel thread's state.

Yup, that's the way to go.

> However this doesn't touch to used_vsr and used_vr which may remain at 0.

That would be a kernel bug.

> 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 ?

Cheers,
Ben.

> Cheers,
> Laurent.
> 
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