Re: [PATCH v1] s390x: Reject unaligned RAM sizes

[Christian Borntraeger]

On 27.03.20 16:29, David Hildenbrand wrote:
> Historically, we fixed up the RAM size (rounded it down), to fit into
> storage increments. Since commit 3a12fc61af5c ("390x/s390-virtio-ccw: use
> memdev for RAM"), we no longer consider the fixed-up size when
> allcoating the RAM block - which will break migration.
> 
> Let's simply drop that manual fixup code and let the user supply sane
> RAM sizes. This will bail out early when trying to migrate (and make
> an existing guest with e.g., 12345 MB non-migratable), but maybe we
> should have rejected such RAM sizes right from the beginning.
> 
> As we no longer fixup maxram_size as well, make other users use ram_size
> instead. Keep using maxram_size when setting the maximum ram size in KVM,
> as that will come in handy in the future when supporting memory hotplug
> (in contrast, storage keys and storage attributes for hotplugged memory
>  will have to be migrated per RAM block in the future).
> 
> This fixes (or rather rejects early):
> 
> 1. Migrating older QEMU to upstream QEMU (e.g., with "-m 1235M"), as the
>    RAM block size changed.

Not sure I like this variant. Instead of breaking migration (that was 
accidentially done by Igors changes) we now reject migration from older
QEMUs to 5.0. This is not going to help those that still have such guests
running and want to migrate. 

> 
> 2. Migrating upstream QEMU to upstream QEMU (e.g., with "-m 1235M"), as
>    we receive storage attributes for memory we don't expect (as we fixed up
>    ram_size and maxram_size).
> 
> Fixes: 3a12fc61af5c ("390x/s390-virtio-ccw: use memdev for RAM")
> Reported-by: Lukáš Doktor <address@hidden>
> Cc: Igor Mammedov <address@hidden>
> Cc: Dr. David Alan Gilbert <address@hidden>
> Signed-off-by: David Hildenbrand <address@hidden>