Re: [Qemu-devel] [RFC PATCH v0 0/2] Increase max memslots to 512 for PowerPC

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

On Thu, Jun 02, 2016 at 09:03:15AM +0200, Thomas Huth wrote:
> On 02.06.2016 06:39, Bharata B Rao wrote:
> ...
> > Agreed. Here is the updated patch:
> > 
> > spapr: Increase hotpluggable memory slots to 256
> > 
> > From: Bharata B Rao <bharata@linux.vnet.ibm.com>
> > 
> > KVM now supports 512 memslots on PowerPC (earlier it was 32). Allow half
> > of it (256) to be used as hotpluggable memory slots.
> > 
> > Instead of hard coding the max value, use the KVM supplied value if KVM
> > is enabled. Otherwise resort to the default value of 32.
> > 
> > Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> > ---
> >  hw/ppc/spapr.c |   15 +++++++++++++--
> >  1 file changed, 13 insertions(+), 2 deletions(-)
> > 
> > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > index 44e401a..c82adef 100644
> > --- a/hw/ppc/spapr.c
> > +++ b/hw/ppc/spapr.c
> > @@ -1816,11 +1816,22 @@ static void ppc_spapr_init(MachineState *machine)
> >      /* initialize hotplug memory address space */
> >      if (machine->ram_size < machine->maxram_size) {
> >          ram_addr_t hotplug_mem_size = machine->maxram_size - machine->ram_size;
> > +        /*
> > +         * Number of memslots supported by KVM on PowerPC was increased
> > +         * from 32 to 512. Let us limit the number of hotpluggable slots
> > +         * to half of that (256). However ensure that number of slots
> > +         * doesn't drop below 32 on older hosts.
> > +         */
> 
> Using "hard-coded" information like "increased to 512" in comments is
> true for the current state, but this has a risk of being out of date
> soon. Once we change the memslots in the kernel, this comment is not
> true anymore and might cause confusion. Better talk about leaving half
> of the kernel memslots for PCI and other devices, or so.

Just want to note that even though we are limiting hotpluggable memory
slots to half of max, it is always possible for other devices to eat
into the memory hotplug slots, right ?

David - here is the patch updated with comments as suggested by Thomas.
If you need a separate post, let me know.

spapr: Increase hotpluggable memory slots to 256

From: Bharata B Rao <bharata@linux.vnet.ibm.com>

KVM now supports 512 memslots on PowerPC (earlier it was 32). Allow half
of it (256) to be used as hotpluggable memory slots.

Instead of hard coding the max value, use the KVM supplied value if KVM
is enabled. Otherwise resort to the default value of 32.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
---
 hw/ppc/spapr.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 44e401a..14cc6ae 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -1816,11 +1816,21 @@ static void ppc_spapr_init(MachineState *machine)
     /* initialize hotplug memory address space */
     if (machine->ram_size < machine->maxram_size) {
         ram_addr_t hotplug_mem_size = machine->maxram_size - machine->ram_size;
+        /*
+         * Limit the number of hotpluggable memory slots to half the number
+         * slots that KVM supports, leaving the other half for PCI and other
+         * devices. However ensure that number of slots doesn't drop below 32.
+         */
+        int max_memslots = kvm_enabled() ? kvm_get_max_memslots() / 2 :
+                           SPAPR_MAX_RAM_SLOTS;
 
-        if (machine->ram_slots > SPAPR_MAX_RAM_SLOTS) {
+        if (max_memslots < SPAPR_MAX_RAM_SLOTS) {
+            max_memslots = SPAPR_MAX_RAM_SLOTS;
+        }
+        if (machine->ram_slots > max_memslots) {
             error_report("Specified number of memory slots %"
                          PRIu64" exceeds max supported %d",
-                         machine->ram_slots, SPAPR_MAX_RAM_SLOTS);
+                         machine->ram_slots, max_memslots);
             exit(1);
         }