Re: [x86, vdso] BUG: unable to handle kernel paging request at d34bd000

[Andy Lutomirski]

On Mon, Mar 10, 2014 at 2:20 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Mar 10, 2014 at 1:19 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> If the only immediate problem is the code generation size, then Andy
>> already had a (simpler) hack-around:
>>
>>   #undef CONFIG_OPTIMIZE_INLINING
>>   #undef CONFIG_X86_PPRO_FENCE
>>
>> in vclock_gettime.c
>
> Btw, we should seriously consider getting rid of CONFIG_X86_PPRO_FENCE.
>
> It was of questionable value to begin with, and I think that the
> actual PPro bug is about one of
>
>  - Errata 66, "Delayed line invalidation".
>  - Errata 92, "Potential loss of data coherency"
>
> both of which affect all PPro versions afaik (there is also a UP
> errata 51 wrt ordering of cached and uncached accesses that was fixed
> in the sB1 stepping).
>
> And as far as I know, we have never actually seen the bug in real
> life, EVEN WHEN PPRO WAS COMMON. The workaround was always based on
> knowledge of the errata afaik.

I admit I don't fully follow the description of the errata, but it's
not obvious to me that making smp_rmb() emit lfence is going to do any
good.  The description seems to be suggesting using actual LOCK
operations to work around the erratum.

>
> So I do think we might want to consider retiring that config option
> entirely as a "historical oddity".
>
> And very much so for the vdso case. Do we even do the asm alternative
> fixups for the vdso?

Yes, we've done that for a couple years for rdtsc_barrier's benefit.

>
> I also suspect we should get rid of CONFIG_X86_OOSTORE, or at least
> limit it to !SMP - I don't think anybody ever made SMP systems with
> those IDT/Centaur Winchip chips in them.

Why does OOSTORE matter for !SMP?  Is it just for poking at hardware registers?

--Andy
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