Re: [Qemu-devel] [PATCH] stop cpus before forking.

[Glauber Costa <glommer@redhat.com>]

On Tue, Jun 15, 2010 at 10:33:21AM +0300, Avi Kivity wrote:
> On 06/14/2010 10:33 PM, Anthony Liguori wrote:
> >On 06/14/2010 02:27 PM, Glauber Costa wrote:
> >>This patch fixes a bug that happens with kvm, irqchip-in-kernel,
> >>while adding a netdev. Despite the situations of reproduction being
> >>specific to kvm, I believe this fix is pretty generic, and fits here.
> >>Specially if we ever want to have our own irqchip in kernel too.
> >>
> >>The problem happens after the fork system call, and although it is not
> >>100 % reproduceable, happens pretty often. After fork, the memory where
> >>the apic is mapped is present in both processes. It ends up confusing
> >>the vcpus somewhere in the irq<->  ack path, and qemu hangs, with no
> >>irqs being delivered at all from that point on.
> >>
> >>Making sure the vcpus are stopped before forking makes the problem go
> >>away. Besides, this is a pretty unfrequent operation, which
> >>already hangs
> >>the io-thread for a while. So it should not hurt performance.
> >
> >This doesn't make very much sense to me but smells like a kernel
> >bug to me.
> 
> It is, and the fix would be to create the APIC memory slot as
> sharable across forks (should be easy to fix in the kernel).
Kernel pages are already shared across fork, no?