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

[David Gibson <david@gibson.dropbear.id.au>]

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.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 hw/ppc/spapr.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index f6e9c2a..9f3e004 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -2376,12 +2376,15 @@ static void spapr_phb_placement(sPAPRMachineState *spapr, uint32_t index,
                                 unsigned n_dma, uint32_t *liobns, Error **errp)
 {
     const uint64_t base_buid = 0x800000020000000ULL;
-    const hwaddr phb0_base = 0x10000000000ULL; /* 1 TiB */
     const hwaddr phb_spacing = 0x1000000000ULL; /* 64 GiB */
     const hwaddr mmio_offset = 0xa0000000; /* 2 GiB + 512 MiB */
     const hwaddr pio_offset = 0x80000000; /* 2 GiB */
     const uint32_t max_index = 255;
+    const hwaddr phb0_alignment = 0x10000000000ULL; /* 1 TiB */
 
+    uint64_t max_hotplug_addr = spapr->hotplug_memory.base +
+        memory_region_size(&spapr->hotplug_memory.mr);
+    hwaddr phb0_base = QEMU_ALIGN_UP(max_hotplug_addr, phb0_alignment);
     hwaddr phb_base;
     int i;
 
-- 
2.7.4