Messages in this thread | | | Subject | Re: [v4] powerpc: Export thread_struct.used_vr/used_vsr to user space | From | Benjamin Herrenschmidt <> | Date | Thu, 07 Jul 2016 23:21:18 +1000 |
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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. > > _______________________________________________ > Linuxppc-dev mailing list > Linuxppc-dev@lists.ozlabs.org > https://lists.ozlabs.org/listinfo/linuxppc-dev
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