Messages in this thread | | | Subject | Re: [v4] powerpc: Export thread_struct.used_vr/used_vsr to user space | From | Laurent Dufour <> | Date | Thu, 7 Jul 2016 15:12:20 +0200 |
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On 07/07/2016 13:15, Michael Ellerman wrote: > On Thu, 2016-07-07 at 07:49:36 UTC, Simon Guo wrote: >> From: Simon Guo <wei.guo.simon@gmail.com> >> >> These 2 fields track whether user process has used Altivec/VSX >> registers or not. They are used by kernel to setup signal frame >> on user stack correctly regarding vector part. > > I still dislike this. It's just exporting internal kernel state, which I know is > the point. > > And I'd still like to know why we're the only arch that needs to do this. > > I'm not saying I won't merge it, but I'd like to understand it better first. > >> CRIU(Checkpoint and Restore In User space) builds signal frame >> for restored process. It will need this export information to >> setup signal frame correctly. And CRIU will need to restore these >> 2 fields for the restored process. > > I don't really know how CRIU works, but .. > > Does the kernel write a sigframe for the process that's being checkpointed? If > so can't you infer the state of the bits based on what was written?
Hi Michael,
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.
> 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. However this doesn't touch to used_vsr and used_vr which may remain at 0.
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.
Cheers, Laurent.
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