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

From: Christian Borntraeger
Subject: Re: [PATCH v1] s390x: Reject unaligned RAM sizes
Date: Fri, 27 Mar 2020 17:05:34 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 27.03.20 17:01, David Hildenbrand wrote:
> On 27.03.20 16:34, Christian Borntraeger wrote:
>> 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. 
> As Igor mentioned on another channel, you most probably can migrate an
> older guest by starting it on the target with a fixed-up size.
> E.g., migrate an old QEMU "-m 1235M" to a new QEMU "-m 1234M"

Yes, that should probably work.
> Not sure how many such weird-size VMs we actually do have in practice.

I am worried about some automated deployments where tooling has created
these sizes for dozens or hundreds of containers in VMS and so.

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