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


From: Avi Kivity
Subject: Re: [Qemu-devel] [PATCH] stop cpus before forking.
Date: Tue, 15 Jun 2010 10:33:21 +0300
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100430 Fedora/3.0.4-3.fc13 Thunderbird/3.0.4

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).

Even if it isn't, I can't rationalize why stopping the vm like this is enough to fix such a problem. Is the problem that the KVM VCPU threads get duplicated while potentially running or something like that?

I think it's COW triggering a copy on one vcpu while the other reads from the old page.

--
error compiling committee.c: too many arguments to function




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