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Subject[RFC PATCH v2 1/1] powerpc/kernel: Enables memory hot-remove after reboot on pseries guests
While providing guests, it's desirable to resize it's memory on demand.

By now, it's possible to do so by creating a guest with a small base
memory, hot-plugging all the rest, and using 'movable_node' kernel
command-line parameter, which puts all hot-plugged memory in
ZONE_MOVABLE, allowing it to be removed whenever needed.

But there is an issue regarding guest reboot:
If memory is hot-plugged, and then the guest is rebooted, all hot-plugged
memory goes to ZONE_NORMAL, which offers no guaranteed hot-removal.
It usually prevents this memory to be hot-removed from the guest.

It's possible to use device-tree information to fix that behavior, as
it stores flags for LMB ranges on ibm,dynamic-memory-vN.
It involves marking each memblock with the correct flags as hotpluggable
memory, which mm/memblock.c puts in ZONE_MOVABLE during boot if
'movable_node' is passed.

For carrying such information, the new flag DRCONF_MEM_HOTPLUGGED is
proposed, which should be true if memory was hot-plugged on guest, and
false if it's base memory.

During boot, guest kernel reads the device-tree, early_init_drmem_lmb()
is called for every added LMBs. Here, checking for this new flag and
marking memblocks as hotplugable memory is enough to get the desirable

This should cause no change if 'movable_node' parameter is not passed
in kernel command-line.

Signed-off-by: Leonardo Bras <>

The new flag was already proposed on Power Architecture documentation,
and it's waiting for approval.

I would like to get your comments on this change, but it's still not
ready for being merged.

I will send the matching qemu change as a reply later.

Changes since v1:
- Adds new flag, so PowerVM is compatible with the change.
- Fixes mistakes in code
arch/powerpc/include/asm/drmem.h | 1 +
arch/powerpc/kernel/prom.c | 9 +++++++--
2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/drmem.h b/arch/powerpc/include/asm/drmem.h
index 3d76e1c388c2..92083b4565f6 100644
--- a/arch/powerpc/include/asm/drmem.h
+++ b/arch/powerpc/include/asm/drmem.h
@@ -65,6 +65,7 @@ struct of_drconf_cell_v2 {
#define DRCONF_MEM_ASSIGNED 0x00000008
#define DRCONF_MEM_AI_INVALID 0x00000040
#define DRCONF_MEM_RESERVED 0x00000080
+#define DRCONF_MEM_HOTPLUGGED 0x00000100

static inline u32 drmem_lmb_size(void)
diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index 6620f37abe73..9c5cb2e8049e 100644
--- a/arch/powerpc/kernel/prom.c
+++ b/arch/powerpc/kernel/prom.c
@@ -515,9 +515,14 @@ static void __init early_init_drmem_lmb(struct drmem_lmb *lmb,
size = 0x80000000ul - base;

+ if (!validate_mem_limit(base, &size))
+ continue;
DBG("Adding: %llx -> %llx\n", base, size);
- if (validate_mem_limit(base, &size))
- memblock_add(base, size);
+ memblock_add(base, size);
+ if (lmb->flags & DRCONF_MEM_HOTPLUGGED)
+ memblock_mark_hotplug(base, size);
} while (--rngs);
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