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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <86fe2845-0804-ff48-cf1b-3bf7e4ff3310@redhat.com>
Date: Wed, 6 Jun 2018 16:23:59 +0200
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Subject: Re: [Qemu-devel] An emulation failure occurs,
 if I hotplug vcpus immediately after the VM start
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To: xuyandong <xuyandong2@huawei.com>, "Gonglei (Arei)" <arei.gonglei@huawei.com>, Igor Mammedov <imammedo@redhat.com>
Cc: Zhanghailiang <zhang.zhanghailiang@huawei.com>, "wangxin (U)" <wangxinxin.wang@huawei.com>, lidonglin <lidonglin@huawei.com>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "Huangweidong (C)" <weidong.huang@huawei.com>

On 06/06/2018 16:18, xuyandong wrote:
>> We don't.  It's generally a guest bug if they do, but the problem here is that
>> QEMU is splitting a memory region in two parts and that is not atomic.
>> 	
>> One fix could be to add a KVM_SET_USER_MEMORY_REGIONS ioctl that
>> replaces the entire memory map atomically.
>>
>> Paolo
> After we add a KVM_SET_USER_MEMORY_REGIONS ioctl that replaces the entire
> memory map atomically, how to use it in address_space_update_topology?
> Shall we checkout the spilt memory region before 
> " address_space_update_topology_pass(as, old_view, new_view, false); 
> address_space_update_topology_pass(as, old_view, new_view, true);

You would add the regions to an array in kvm_region_add, and send the
ioctl in the .commit callback of MemoryListener.

kvm_region_del would disappear.  The .commit callback would also look at
the array from the previous execution, and call memory_region_unref on
the regions in there.

Paolo