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

[Vasilis Liaskovitis]

On Wed, Oct 17, 2012 at 12:03:51PM +0200, Avi Kivity wrote:
> 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.

thanks, I 'll take a look. It sounds like the other options below are more
straightforward now, but let me know if you prefer q35 integration as a 
priority.

> 
> > 
> > 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.

ok

> 
> > 
> > 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.

Do we want this generic scheme above to be tied into the i440fx/pc machine?
Or have it as a separate generic memory bus / pmc usable by others (e.g. in
hw/dimm.c)?
The 64-bit values you describe are already part of DimmDevice properties, but
they are not hardware registers described as part of a chipset.

In terms of control bits, did you want to mimic some other chipset registers? - 
any examples would be useful.

> 
> > 
> > 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.

But if we use the real i440fx pmc DRBs for base memory, this means base memory
would be <= 2GB, right?

Sounds like we 'd need to change the DRBs anyway to describe useful amounts of
base memory (e.g. 512MB chunks and check against address lines [36:29] can
describe base memory up to 64GB, though that's still limiting for very large
VMs). But we'd be diverting from the real hardware again.
    
Then we can model base memory with "tweaked" i440fx pmc's DRB registers - we
could only use DRB[0] (one DIMM describing all of base memory) or more.

DIMMs would be allowed to be hotplugged in the generic mem-controller scheme 
only
(unless it makes sense to allow hotplug in the remaining pmc DRBs and
start using the generic scheme once we run out of emulated DRBs)

thanks,

- Vasilis