[Qemu-devel] [RFC PATCH v0] spapr: Disable memory hotplug when HTAB size is insufficient

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

The hash table size allocated to guest depends on the maxmem size.
If the host isn't able to allocate the required hash table size but
instead allocates less than the optimal requested size, then it will
not be possible to grow the RAM until maxmem via memory hotplug.
Attempts to hotplug memory till maxmem could fail and this failure
isn't being currently handled gracefully by the guest kernel thereby
causing guest kernel oops.

This should eventually get fixed when we move to completely in-kernel
memory hotplug instead of the current method where userspace tool drmgr
drives the hotplug. Until the in-kernel memory hotplug is available
for PowerKVM, disable memory hotplug when requested hash table size
isn't allocated.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
---
Applies against spapr-next branch of David Gibson's tree.

 hw/ppc/spapr.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index c3268c5..4a07a7d 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -92,6 +92,9 @@
 
 #define HTAB_SIZE(spapr)        (1ULL << ((spapr)->htab_shift))
 
+/* TODO: Move this to sPAPRMachineState ? */
+static bool spapr_memory_hotplug_disabled;
+
 static XICSState *try_create_xics(const char *type, int nr_servers,
                                   int nr_irqs, Error **errp)
 {
@@ -983,6 +986,14 @@ static void spapr_reset_htab(sPAPRMachineState *spapr)
 
     if (shift > 0) {
         /* Kernel handles htab, we don't need to allocate one */
+        if (shift != spapr->htab_shift) {
+            /*
+             * Disable memory hotplug since we didn't get the requested
+             * hash table size.
+             */
+            spapr_memory_hotplug_disabled = true;
+        }
+
         spapr->htab_shift = shift;
         kvmppc_kern_htab = true;
 
@@ -2149,6 +2160,11 @@ static void spapr_machine_device_plug(HotplugHandler *hotplug_dev,
             return;
         }
 
+        if (spapr_memory_hotplug_disabled) {
+            error_setg(errp, "Insufficient HTAB size to support memory hotplug");
+            return;
+        }
+
         spapr_memory_plug(hotplug_dev, dev, node, errp);
     }
 }
-- 
2.1.0