[virtio-dev] Re: [Qemu-devel] [PATCH v7 3/5] migration: API to clear bits of guest free pages from the dirty bitmap

[Wei Wang <wei.w.wang@intel.com>]

On 06/04/2018 10:49 AM, Peter Xu wrote:
>
>>
>>
>>>>>> +
>>>>>> +    for (; len > 0; len -= used_len) {
>>>>>> +        block = qemu_ram_block_from_host(addr, false, &offset);
>>>>>> +        if (unlikely(!block)) {
>>>>>> +            return;
>>>>> We should never reach here, should we?  Assuming the callers of this
>>>>> function should always pass in a correct host address. If we are very
>>>>> sure that the host addr should be valid, could we just assert?
>>>> Probably not the case, because of the corner case that the memory would be
>>>> hot unplugged after the free page is reported to QEMU.
>>> Question: Do we allow to do hot plug/unplug for memory during
>>> migration?
>> I think so. From the code, I don't find where it forbids memory hotplug
>> during migration.
> I don't play with that much; do we need to do "device_add" after all?
>
>    (qemu) object_add memory-backend-file,id=mem1,size=1G,mem-path=/mnt/hugepages-1GB
>    (qemu) device_add pc-dimm,id=dimm1,memdev=mem1
>
> If so, we may not allow that since in qdev_device_add() we don't allow
> that:
>
>      if (!migration_is_idle()) {
>          error_setg(errp, "device_add not allowed while migrating");
>          return NULL;
>      }
>

OK, I missed that part, and thanks for correcting it. I'll use an assert 
there if no objections from others.


Best,
Wei



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