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

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] [PATCH 04/10] Translate offsets to destination address space
Date: Tue, 19 May 2015 20:02:11 +0100
User-agent: Mutt/1.5.23 (2014-03-12)

* Michael R. Hines (address@hidden) wrote:
> On 05/19/2015 01:44 PM, Dr. David Alan Gilbert wrote:
> >* Michael R. Hines (address@hidden) wrote:
> >>On 04/20/2015 10:57 AM, Dr. David Alan Gilbert (git) wrote:
> >>>From: "Dr. David Alan Gilbert" <address@hidden>
> >>>
> >>>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 =).


Dr. David Alan Gilbert / address@hidden / Manchester, UK

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