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    SubjectRe: new text patching for review
    * Jeremy Fitzhardinge ( wrote:
    > Andi Kleen wrote:
    > > Mathieu Desnoyers <> 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.
    > > - 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?

    I align the "immediate values" within the mov instructions on multiples
    of the immediate value size so I can update it atomically.

    > J

    Mathieu Desnoyers
    Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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