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

Stephen Hemminger
2009-06-09T18:37:16Z

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

> Dan Williams wrote:
> > On Mon, Jun 8, 2009 at 5:04 PM, Stephen Hemminger<shemminger@vyatta.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@redhat.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@kernel.org
> > Reported-and-tested-by: Yinghai Lu <yinghai@kernel.org>
> > Signed-off-by: Avi Kivity <avi@redhat.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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