Re: new text patching for review

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Jeremy Fitzhardinge (jeremy@goop.org) wrote:
> Andi Kleen wrote:
> > Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> 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