linux-kernel - Re: [Bug #13116] Can't boot with nosmp

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Date:	Tue, 9 Jun 2009 11:36:56 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>
Subject: Re: [Bug #13116] Can't boot with nosmp

On Tue, 09 Jun 2009 21:30:43 +0300
Avi Kivity <avi@...hat.com> wrote:

> Dan Williams wrote:
> > On Mon, Jun 8, 2009 at 5:04 PM, Stephen Hemminger<shemminger@...tta.com> wrote:
> >   
> >> I did some more debugging, and the problem appears to be that
> >> booting with nosmp still leaves all the CPU's present in the cpu
> >> possible mask. So code like dma engine (or loopback device), that
> >> use alloc_percpu get broken.
> >>
> >> Not sure why all the cpu's are still showing up in the possible mask.
> >> Rusty? do you have an answer.
> >>     
> >
> > Would this recent commit also fix this issue?
> >
> > commit a4c0364be3f43d3e17fe19270f8b3d64881606e6
> > Author: Avi Kivity <avi@...hat.com>
> > Date:   Sat Jun 6 12:34:39 2009 +0300
> >
> >     KVM: Explicity initialize cpus_hardware_enabled
> >
> >     Under CONFIG_MAXSMP, cpus_hardware_enabled is allocated from the heap and
> >     not statically initialized.  This causes a crash on reboot when kvm thinks
> >     vmx is enabled on random nonexistent cpus and accesses nonexistent percpu
> >     lists.
> >
> >     Fix by explicitly clearing the variable.
> >
> >     Cc: stable@...nel.org
> >     Reported-and-tested-by: Yinghai Lu <yinghai@...nel.org>
> >     Signed-off-by: Avi Kivity <avi@...hat.com>
> >   
> 
> This only affects reboots.  But yes, the cpumask code has bugs.  Does 
> the .config involved select MAXSMP?
> 

Patch does not fix the problem. Not sure if it is KVM only problem.
Look like possible cpumask always equals what ACPI / SMP tables report.
So currently looking into alloc_percpu which may not handle the possible
but never used cpus.

It is x86-64 config and does not have MAXSMP set. It does have NR_CPUS set.

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