Re: [PATCH] x86: put trigger in to detect mismatched apic versions.

[Ingo Molnar]

* Mike Travis <travis@xxxxxxx> wrote:

> Ingo Molnar wrote:
> > * Mike Travis <travis@xxxxxxx> wrote:
> > 
> >> Hi Ingo,
> >>
> >> I did notice that the versions all came up the same, and that the checks 
> >> were very specific.  I was trying to be as transparent and unintrusive 
> >> as possible.  Since there's so few calls, I though this was a good 
> >> approach but apparently I was wrong.
> >>
> >> I like the idea of collapsing the array down to one and checking to see 
> >> if all apic's have the same version, but is this really the case? Must 
> >> all apics be the same?
> > 
> > Could you please send a patch that doesnt change the code, only adds a 
> > 'boot APIC version' kind of variable as an apic_version __read_mostly 
> > variable and does a WARN_ONCE() if that mismatches? We can then stick that 
> > into -tip and see whether it triggers.
> > 
> > The max array size is ~128K, right? So if the WARN_ONCE() does not 
> > trigger, we can just drop the array and use the central apic_version 
> > variable ...
> > 
> > And even if it _does_ trigger, the version incompatibilities between APIC 
> > protocols are very rare. They only happen across wildly different CPU 
> > architectures like when going from very old external apics to integrated 
> > apics, or going from apics to x-apics. We wont see any mixing across those 
> > boundaries.
> > 
> > 	Ingo
> 
> Btw, I checked with our UV architect and the problem is that we need a 
> 16 bit apic id which is what caused the MAX_APICS to be bumped to 32k. 
> The lower 8 bits are the normal apic id, and the upper bit relate to the 
> node.  This means cpu 0 on node 0 has the same apic id as cpu 0 on node 
> 1, etc.  I also asked about whether we could rely on always having the 
> same apic version, and the answer was yes, though it's really only 
> relevant between the cpus on a node.
> 
> Thanks,
> Mike
> --- 
> Subject: x86: put trigger in to detect mismatched apic versions.
> 
> Fire off one message if two apic's discovered with different
> apic versions.
> 
> Signed-off-by: Mike Travis <travis@xxxxxxx>
> ---
>  arch/x86/kernel/apic.c |    5 +++++
>  1 file changed, 5 insertions(+)

Applied to tip/x86/urgent, thanks Mike! Note: for such changes we want a 
more comprehensive changelog that explains the motivation - i wrote one 
up, see the commit below.

	Ingo

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