Re: [Qemu-arm] [Qemu-devel] [RFC v2 6/6] hw/arm: Populate non-contiguous memory regions

[Andrew Jones]

On Tue, Jun 05, 2018 at 07:49:44AM +0000, Shameerali Kolothum Thodi wrote:
> > > On 05/16/2018 05:20 PM, Shameer Kolothum wrote:
> > > > In case valid iova regions are non-contiguous, split the
> > > > RAM mem into a 1GB non-pluggable dimm and remaining as a
> > > > single pc-dimm mem.
> > >
> > > Please can you explain where does this split come from? Currently we
> > > have 254 GB non pluggable RAM. I read the discussion started with Drew
> > > on RFC v1 where he explained we cannot change the RAM base without
> > > crashing the FW. However we should at least document this  1GB split.
> > 
> > The comments were mainly to use the pc-dimm model instead of "mem alias"
> > method used on RFC v1 as this will help to add the mem hot-plug support
> > in future.
> > 
> > I am not sure about the motive behind Drew's idea of splitting the first
> > 1GB as non-plug and remaining as a pc-dimm(cold). May it is to attempt a
> > best effort scenario, but as I mentioned in reply to 0/6, the current 
> > solution
> > will end up changing the base address if the 0x4000000 falls under a 
> > reserved
> > region.
> > 
> > Again, not sure whether we should enforce a strict check on base address
> > start or just warn the user that it will fail on Guest with UEFI boot[1].
> > 
> > Hi Drew,
> > 
> > Please let me know your thoughts on this.
> 
> Could you please take a look at the above discussion and let us
> know your thoughts on the split of mem regions as 1GB non-pluggable
> and rest as pc-dimm.
> 

Hi Shameer,

Sorry for the slow reply - I've been slowly catching back up after
vacation. There are two reasons to have one non-pluggable memory region
and the rest of memory in a single pluggable pc-dimm.

1) For mach-virt we must have the base of memory at the 1G boundary,
   both because otherwise we'll break ArmVirt UEFI and because that's
   a guarantee that Peter has stated he'd like to keep for mach-virt.

2) Memory split in this way already has precedent in the x86 PC
   machine models.

It's debatable how much memory we should allocate to the non-pluggable
region. It doesn't need to be 1G, it could be smaller. The default
memory size allocated to a mach-virt guest that doesn't provide '-m'
on the command line is 128M. Maybe we should use that size?

If 0x40000000 falls into a reserved address region on some host, then
I'm afraid the mach-virt model won't work with those devices unless the
guest doesn't use UEFI and Peter is open to providing a machine property
that one can enable in order to move the base address of memory.

I know Eric is looking at this problem too. I hope you guys are
coordinating your efforts.

Thanks,
drew