Re: [Qemu-devel] Segfault with coalesced mmio and boot CPU removal

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Jul 27, 2016 at 10:01:41AM +0530, Bharata B Rao wrote:
> On Wed, Jul 27, 2016 at 08:22:51AM +0530, Bharata B Rao wrote:
> > Hi,
> > 
> > Coalesced mmio buffer is part of vCPU 0's kvm_run mmap'ed area
> > and with the introduction of CPU hotplug, vCPU 0 can be removed on
> > PowerPC leading to the below seen segfault in QEMU.
> > 
> > 
> > This happens because during CPU removal, though we park the kvm_fd
> > corresponding to the removed vCPU thread, we unmap the kvm_run (and
> > hence coalesced mmio ring).
> > 
> <snip>
> > What would be the best way to fix this ? Is disassociating coalesced_mmio_ring
> > from vCPU 0's kvm_run the correct solution ?
> 
> May be PowerPC too should do what x86 does like below for now ?

I think that's what we need to do short term.  In the 2.8 timeframe,
separating the mmio ring out from the vcpu state sounds like a good
idea, but I don't really know how complicated that will be.

> 
> commit 73360e27850b213327011f7e22e03865b8c0dd5b
> Author: Igor Mammedov <imammedo@redhat.com>
> Date:   Mon Jul 18 10:31:22 2016 +0200
> 
>     pc: Forbid BSP removal
>     
>     Boot CPU is assumed to always present in QEMU code, so
>     untile that assumptions are gone, deny removal request,
>     In another words QEMU won't support BSP hot-unplug.
> 

-- 
David Gibson			| I'll have my music baroque, and my code
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