Re: [patches] new text patching for review

[Andi Kleen <ak@suse.de>]

On Thursday 19 July 2007 22:30:12 Jeremy Fitzhardinge 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.

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.

-Andi