Re: [Qemu-devel] [PATCH 04/10] Translate offsets to destination address space

["Michael R. Hines" <mrhines@linux.vnet.ibm.com>]

On 05/19/2015 01:44 PM, Dr. David Alan Gilbert wrote:
> * Michael R. Hines (mrhines@linux.vnet.ibm.com) wrote:
>> On 04/20/2015 10:57 AM, Dr. David Alan Gilbert (git) wrote:
>>> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>>>
>>> The 'offset' field in RDMACompress and 'current_addr' field
>>> in RDMARegister are commented as being offsets within a particular
>>> RAMBlock, however they appear to actually be offsets within the
>>> ram_addr_t space.
>>>
>>> The code currently assumes that the offsets on the source/destination
>>> match, this change removes the need for the assumption for these
>>> structures by translating the addresses into the ram_addr_t space of
>>> the destination host.
>> I don't understand fully: If the offsets are not the same, then
>> why would the RAMBlocks be the same? If a RAMBlock
>> is hot-plugged on one side, shouldn't an identical one be
>> hotplugged on the other side, including the offset into ram_addr_t?
> If a RAMBlock is hotplugged on the source, it's normally passed in on
> the command line at startup on the destination, not hotplugged.
>
> This difference in order of allocation of the RAMBlocks can cause
> the allocation of space in ram_addr_t to be different.
>
> In addition changes between qemu versions as to the order in which
> devices are initialised can cause differences in the allocation in ram_addr_t.
>
> Indeed the allocation algorithm isn't very deterministic, I think it looks
> for the smallest gap that will fit for the allocation.
> Also, lets say that you hot plugged 6 devices, and hot unplugged 5,
> then migrated;  on the destination you only see one of them, not the history
> of the other allocations that had to be performed.

I see ---- so until now, I just got "lucky" that the RAMBlocks were
in the same order with the same offsets =).