Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu area

[Eric W. Biederman]

Mike Travis <travis@xxxxxxx> writes:

> H. Peter Anvin wrote:
>> Jeremy Fitzhardinge wrote:
>>>
>>> Yes, and there's no reason we couldn't do the same on 64-bit, aside
>>> from the stack-protector's use of %gs:40.  There's no code-size cost
>>> in large offsets, since they're always 32-bits anyway (there's no
>>> short absolute addressing mode).
>>>
>>> If we manually generate %gs-relative references to percpu data, then
>>> it's no different to what we do with 32-bit, whether it be a specific
>>> symbol address or using the TLS relocations.
>>>
>> 
>> If we think the problem is the zero-basing triggering linker bugs, we
>> should probably just use a small offset, like 64 (put a small dummy
>> section before the .percpu.data section to occupy this section.)
>> 
>> I'm going to play with this a bit and see if I come up with something
>> sanish.
>> 
>>     -hpa
>
> One interesting thing I've discovered is the gcc --version may make a
> difference.
>
> The kernel panic that occurred from Ingo's config, I was able to replicate
> with GCC 4.2.0 (which is on our devel server).  But this one complained
> about not being able to handle the STACK-PROTECTOR option so I moved
> everything to another machine that has 4.2.4, and now it seems that it
> works fine.  I'm still re-verifying that the source bits and config options 
> are identical (it was a later git-remote update), and that in fact it is
> the gcc --version, but that may be the conclusion.  (My code also has some
> patches submitted but not yet included in the tip/master tree.  Curiously
> just enabling some debug options changed the footprint of the panic.)
>
> Are we allowed to insist on a specific level of GCC for compiling the
> kernel?

Depends on the root cause.  If it turns out to be something that is buggy
in gcc and we can't work around.  We might do something.  I don't recall
that kind of thing happening often.  I think our minimum gcc is currently
gcc-3.4.

Eric
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