* Andi Kleen ([EMAIL PROTECTED]) wrote: > On Thursday 19 July 2007 22:30:12 Jeremy Fitzhardinge wrote: > > Andi Kleen wrote: > > > Mathieu Desnoyers <[EMAIL PROTECTED]> writes: > > > > > >> I see that IRQs are disabled in alternative_instructions(), but it does > > >> not protect against NMIs, which could come at a very inappropriate > > >> moment. MCE and SMIs would potentially cause the same kind of trouble. > > >> > > >> So unless you can guarantee that any code from NMI handler won't call > > >> basic things such as get_cycles() (nor MCE, nor SMIs), you can't insure > > >> it won't execute an illegal instruction. Also, the option of temporarily > > >> disabling the NMI for the duration of the update simply adds unwanted > > >> latency to the NMI handler which could be unacceptable in some setups. > > >> > > > > > > Ok it's a fair point. But how would you address it ? > > > > > > Even if we IPIed the other CPUs NMIs or MCEs could still happen. > > > > > > BTW Jeremy, have you ever considered that problem with paravirt ops > > > patching? > > > > > > > I remember Zach was thinking about it when he was thinking of making vmi > > a kernel module, but I don't think we discussed it with respect to the > > current patching mechanism. Though he did discover that at one point > > alternative_instructions() was being run with interrupts enabled, which > > caused surprisingly few problems... > > > > But, yeah, it seems like it could be a problem. > > Normally there are not that many NMIs or MCEs at boot, but it would > be still good to avoid the very rare crash by auditing the code first > [better than try to debug it on some production system later] > > > > - smp lock patching only ever changes a single byte (lock prefix) of > > > a single instruction > > > - kprobes only ever change a single byte > > > > > > For the immediate value patching it also cannot happen because > > > you'll never modify multiple instructions and all immediate values > > > can be changed atomically. > > > > > > > Are misaligned/cross-cache-line updates atomic? > > In theory yes, in practice there can be errata of course. There tend > to be a couple with self modifying code, especially cross modifying > (from another CPU) -- but you don't do that. >
Hrm, changing instructions in multiple memory accesses does not seem to be atomic to me (unaligned case). -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
