On Wed, 1 Apr 2020 04:50:14 -0400
Christian Borntraeger <borntrae...@de.ibm.com> wrote:
> Older QEMU versions did fixup the ram size to match what can be reported
> via sclp. We need to mimic this behaviour for machine types 4.2 and
> older to not fail on inbound migration for memory sizes that do not fit.
> Old machines with proper aligned memory sizes are not affected.
>
> Alignment table:
> VM size (<=) | Alignment
> --------------------------
> 1020M | 1M
> 2040M | 2M
> 4080M | 4M
> 8160M | 8M
> 16320M | 16M
> 32640M | 32M
> 65280M | 64M
> 130560M | 128M
> 261120M | 256M
> 522240M | 512M
> 1044480M | 1G
> 2088960M | 2G
> 4177920M | 4G
> 8355840M | 8G
>
> Suggested action is to replace unaligned -m value with a suitable
"to replace any unaligned -m value" ?
> aligned one or to use a machine version >= 5.0 as future versions might
> remove the compatibility handling.
I'm confused by the second part of the sentence. Warning about possible
future removal of the compat stuff is fine, but I don't understand the
suggestion to use a machine type >= 5.0. If I create a new machine that
does not need be migrated to an old QEMU, using the latest machine type
always seems like the best idea, right? And for a migration target it's
not like we can choose the version freely anyway.
>
> For machine types >= 5.0 we can simply use an increment size of 1M and
> use the full range of increment number which allows for all possible
> memory sizes. The old limitation of having a maximum of 1020 increments
> was added for standby memory, which we no longer support. With that we
> can now support even weird memory sizes like 10001234 MB.
>
> Fixes: 3a12fc61af5c ("390x/s390-virtio-ccw: use memdev for RAM")
> Reported-by: Lukáš Doktor <ldok...@redhat.com>
> Cc: Igor Mammedov <imamm...@redhat.com>
> Cc: Dr. David Alan Gilbert <dgilb...@redhat.com>
> Signed-off-by: David Hildenbrand <da...@redhat.com>
> Signed-off-by: Christian Borntraeger <borntrae...@de.ibm.com>
> ---
> hw/s390x/s390-skeys.c | 2 +-
> hw/s390x/s390-stattrib-kvm.c | 4 ++--
> hw/s390x/s390-virtio-ccw.c | 19 +++++++++++++++++++
> hw/s390x/sclp.c | 19 ++++++-------------
> include/hw/boards.h | 7 +++++++
> softmmu/vl.c | 3 +++
> 6 files changed, 38 insertions(+), 16 deletions(-)
>
> diff --git a/include/hw/boards.h b/include/hw/boards.h
> index 236d239c19..0532143327 100644
> --- a/include/hw/boards.h
> +++ b/include/hw/boards.h
> @@ -152,6 +152,12 @@ typedef struct {
> * It also will be used as a way to optin into "-m" option support.
> * If it's not set by board, '-m' will be ignored and generic code will
> * not create default RAM MemoryRegion.
> + * @fixup_ram_size:
> + * amends user provided ram size (with -m option) using machine
> + * specific algorithm. to be used by old machine types for compat
> + * purposes only.
> + * Applies only to default memory backend, i.e. explicit memory backend
> + * wasn't used.
"Applies only to the default memory backend, i.e., an explicitly
specified memory backend will not be affected."
?
> */
> struct MachineClass {
> /*< private >*/
