Re: 4.14.9 with CONFIG_MCORE2 fails to boot

[Linus Torvalds]

On Fri, Dec 29, 2017 at 9:32 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> From the various oopses, it looks like this happens when getting a
> double fault while trying to go idle.  The CPU gets is probably trying
> to return from the double fault, but it didn't do anything useful in the
> fault handler so it just continues faulting, but the NMI watchdog can
> still get an oops out of it.

Hmm. Which oops are you looking at? The ones I see in the bugzilla
don't seem to have anything interesting in them.

[ Oh. I think I see the one you think of in the gentoo bug report ]

There does seem to be a lot of odd double faults that don't make progress.

And that in turn indicates that it may be about ESPFIX64 - all other
double fault cases should cause a fault printout, but ESPFIX64 has a
magical silent "turn double fault into a fake #GP fault".

Maybe that one triggers over and over again?

> Couple more things:
>
> MCORE2 seems to get one oddball compiler flag (-march=core2):
>
>>         cflags-$(CONFIG_MCORE2) += \
>>                 $(call cc-option,-march=core2,$(call cc-option,-mtune=generic))
>
> It would be interesting to see if replacing the above "$(call" with:
>
>         $(call cc-option,-mtune=generic)
>
> makes the problem go away the same way as changing the .config option.

Definitely.

> The MCORE2 config option also sets CONFIG_X86_P6_NOP, which overrides
> the normal X86_64 noops, if I'm reading that code correctly.

Only for the ASM_NOPx nops, as far as I can tell. The actual
alternative NOP rewriting seems to pick the nops based on machine, not
on config options.

And I don't see anybody who actually uses the ASM_NOPx defines except
for arch/x86/kernel/kprobes/opt.c, which uses ASM_NOP5.

Am I missing something? We actually have a lot of lines in
arch/x86/include/asm/nops.h that set the ASM_NOPx values to the proper
things, but then they are never used. We have that special
"ASM_NOP5_ATOMIC" define that we are so careful about, but again, it's
actually never used as far as I can tell.

Maybe there's some magic token concatenation use that I'm missing in
my trivial grep, but it does seem to be dead code.

But double-checking that "-march=core2" case is definitely worth
looking into. Especially since there are clear indications that it's
gcc version-dependent anyway. Alexander?

                  Linus