Re: new text patching for review

[Jeremy Fitzhardinge]

Andi Kleen wrote:
> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> 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?

    J
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