Re: [Qemu-devel] [RFC PATCH v3 06/19] Implement "-dimm" command line option

[Avi Kivity]

On 10/17/2012 11:19 AM, Vasilis Liaskovitis wrote:
>> 
>> I don't think so, but probably there's a limit of DIMMs that real
>> controllers have, something like 8 max.
> 
> In the case of i440fx specifically, do you mean that we should model the DRB
> (Dram row boundary registers in section 3.2.19 of the i440fx spec) ?
> 
> The i440fx DRB registers only supports up to 8 DRAM rows (let's say 1 row
> maps 1-1 to a DimmDevice for this discussion) and only supports up to 2GB of
> memory afaict (bit 31 and above is ignored).
> 
> I 'd rather not model this part of the i440fx - having only 8 DIMMs seems too
> restrictive. The rest of the patchset supports up to 255 DIMMs so it would be 
> a
> waste imho to model an old pc memory controller that only supports 8 DIMMs.
> 
> There was also an old discussion about i440fx modeling here:
> https://lists.nongnu.org/archive/html/qemu-devel/2011-07/msg02705.html
> the general direction was that i440fx is too old and we don't want to 
> precisely
> emulate the DRB registers, since they lack flexibility.
> 
> Possible solutions:
> 
> 1) is there a newer and more flexible chipset that we could model?

Look for q35 on this list.

> 
> 2) model and document 
                 ^--- the critical bit

> a generic (non-existent) i440fx that would support more
> and larger DIMMs. E.g. support 255 DIMMs. If we want to use a description
> similar to the i440fx DRB registers, the registers would take up a lot of 
> space.
> In i440fx there is one 8-bit DRB register per DIMM, and DRB[i] describes how
> many 8MB chunks are contained in DIMMs 0...i. So, the register values are
> cumulative (and total described memory cannot exceed 256x8MB = 2GB)

Our i440fx has already been extended by support for pci and cpu hotplug,
and I see no reason not to extend it for memory.  We can allocate extra
mmio space for registers if needed.  Usually I'm against this sort of
thing, but in this case we don't have much choice.

> 
> We could for example model: 
> - an 8-bit non-cumulative register for each DIMM, denoting how many
> 128MB chunks it contains. This allowes 32GB for each DIMM, and with 255 DIMMs 
> we
> describe a bit less than 8TB. These registers require 255 bytes.
> - a 16-bit cumulative register for each DIMM again for 128MB chunks. This 
> allows
> us to describe 8TB of memory (but the registers take up double the space, 
> because
> they describe cumulative memory amounts)

There is no reason to save space.  Why not have two 64-bit registers per
DIMM, one describing the size and the other the base address, both in
bytes?  Use a few low order bits for control.

> 
> 3) let everything be handled/abstracted by dimmbus - the chipset DRB modelling
> is not done (at least for i440fx, other machines could). This is the least 
> precise
> in terms of emulation. On the other hand, if we are not really trying to 
> emulate
> the real (too restrictive) hardware, does it matter?

We could emulate base memory using the chipset, and extra memory using
the scheme above.  This allows guests that are tied to the chipset to
work, and guests that have more awareness (seabios) to use the extra
features.

-- 
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