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

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

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 ?

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.