* Ingo Molnar (mingo@xxxxxxx) wrote: > > * Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote: > > > Just for the sake of making NMI handlers less tricky, supporting > > page faults caused by faulting kernel instructions (rather than > > only supporting explicit faulting from get_user_pages_inatomic) > > would be rather nice design-wise if it only costs 2-3 cycles. > > > > And I would not want to touch the page fault handler itself to > > write the saved cr2 value before the handler exits, because this > > would add a branch on a very hot path. > > _That_ path is not hot at all - it's the 'we are in atomic section > and faulted' rare path (laced with an exception table search - which > is extremely slow compared to other bits of the pagefault path). > > But ... it's not an issue: a check can be made in the NMI code too, > as we always know about pagefaults there, by virtue of getting > -EFAULT back from the attempted-user-copy. As the maintainer of the out-of-tree LTTng tracer, which hooks in the page fault handler with tracepoints, and which can build almost entirely as modules, I am very tempted to argue that having the nmi-code entirely robust wrt in-kernel page faults would be a very-nice-to-have feature. Requiring that code to be either built-in or to call vmalloc_sync_all() after any vmalloc or after module load is just painful and error-prone. Plus, tracing is a feature that some users will only use in specific occasions. I don't see why it should be built-into the kernel at all. That's just a waste of memory in many cases. (I am not talking about users who want to do continuous system tracing here, which is a totally different scenario). Mathieu > > Ingo -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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