Re: [Qemu-devel] [RFC 2/4] spapr: Adjust placement of PCI host bridge to allow > 1TiB RAM

[Laurent Vivier <lvivier@redhat.com>]

On 06/10/2016 05:03, David Gibson wrote:
> Currently the default PCI host bridge for the 'pseries' machine type is
> constructed with its IO windows in the 1TiB..(1TiB + 64GiB) range in
> guest memory space.  This means that if > 1TiB of guest RAM is specified,
> the RAM will collide with the PCI IO windows, causing serious problems.
> 
> Problems won't be obvious until guest RAM goes a bit beyond 1TiB, because
> there's a little unused space at the bottom of the area reserved for PCI,
> but essentially this means that > 1TiB of RAM has never worked with the
> pseries machine type.
> 
> This patch fixes this by altering the placement of PHBs on large-RAM VMs.
> Instead of always placing the first PHB at 1TiB, it is placed at the next
> 1 TiB boundary after the maximum RAM address.
> 
> Technically, this changes behaviour in a migration-breaking way for
> existing machines with > 1TiB maximum memory, but since having > 1 TiB
> memory was broken anyway, this seems like a reasonable trade-off.

Perhaps you can add an SPAPR_COMPAT_XX property with PHB0 base to not
break compatibility? I think spapr without PCI card (only VIO, for
instance), should work with 1TiB+.

On another side, how the SPAPR kernel manages memory?
Is it possible to add an hole in RAM between 1TiB and 1TiB+64GiB to
allow the kernel to register the I/O space?

Laurent