* Ingo Molnar (mingo@xxxxxxx) wrote: > > * Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote: > > > Hrm, would it be possible to save the c2 register upon nmi handler > > entry and restore it before iret instead ? This would ensure a > > nmi-interrupted page fault handler would continue what it was > > doing with a non-corrupted cr2 register after returning from nmi. > > > > Plus, this involves no modification to the page fault handler fast > > path. > > I guess this kind of nesting would work too - assuming the cr2 can > be written to robustly. > > And i suspect CPU makers pull off a few tricks to stage the cr2 info > away from the page fault entry execution asynchronously, so i'd not > be surprised if writing to it uncovered unknown-so-far side-effects > in CPU implementations. > > If possible i wouldnt want to rely on such a narrowly possible hack > really - any small change in CPU specs could cause problems years > down the line. > > The GUP based method is pretty generic though - and can be used on > other architectures as well. It's not as fast as direct access > though. > > Ingo I guess. However, having the ability to call module code in NMI handler context without having to fear for page fault handler re-entrancy (on x86 32) seems like an interesting overall simplification of nmi-handler rules. It is currently far from trivial to write code aimed at NMI handler context. I mean.. LTTng should not have to run vmalloc_sync_all() after loading its modules as it currently does. Maybe it would be worth trying the save/restore cr2 approach and test to figure out how a large variety of machines react. The fact is that hypervisor code already writes into the cr2 register : kvm/vmx.c : vmx_vcpu_run() ... "mov %%"R"ax, %%cr2 \n\t" Mathieu -- 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