Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start

[Paolo Bonzini]

On 07/06/2018 13:36, David Hildenbrand wrote:
>> The dirty bitmap would be synced in kvm_region_del (so it's not true
>> that kvm_region_del would disappear, but almost :)).
> I was rather concerned when doing a KVM_SET_USER_MEMORY_REGIONS while
> some (already present) memory region is performing dirty tracking and
> therefore has a dirty_bitmap pointer assigned.
> 
> As we have to expect that all different kinds of parameters can change
> (e.g. the size of a slot as I pointed out), the old bitmap cannot be
> reused. At least not atomically -  we could create a new one and the
> simply or the old content.
> 
> Well, we could make that dirty tracking a special case ("all dirty
> tracking data will be lost in case doing a KVM_SET_USER_MEMORY_REGIONS"
> - but this again could lead to races (if the bitmap sync happens before
> KVM_SET_USER_MEMORY_REGIONS)). It's tricky to get it right :)

At the point where QEMU calls region_del, the guest is not supposed to
access the region anymore, so it's okay to do the final sync there.  The
same race exists now already.

Paolo

>> The rmap is more interesting.  Perhaps it can be just rebuilt on every
>> KVM_SET_USER_MEMORY_REGIONS call.