[PATCH 00/10] Introduce virtio-mem <memory/> model

David Hildenbrand david at redhat.com
Mon Jan 25 13:03:29 UTC 2021

On 22.01.21 23:03, Daniel Henrique Barboza wrote:
> On 1/22/21 6:19 PM, David Hildenbrand wrote:
>>> Out of curiosity: are you aware of anyone working in enabling virtio-mem
>>> for pseries/ppc64? I'm wondering if there's some kind of architecture
>>> limitation in Power or if it's just a lack of interest.
>> I remember there is interest, however:
>> - arm64 and x86-64 is used more frequently in applicable (cloud?) setups, so it has high prio
>> - s390x doesn‘t have any proper memory hot(un)plug, and as I have a strong s399x background, it‘s rather easy for me to implement
>> - ppc64 at least supports hot(un)plug of DIMMs
>> There is nothing fundamental speaking against ppc64 support AFAIR.
> That's good to hear.
>> A block size of 16MB should be possible. I‘m planning on looking into it, however, there are a lot of other things on my todo list for virtio-mem.
> I'm not familiar with the 'block size' concept of the virtio-mem device that would
> allow for 16MB increments. My knowledge of the pseries kernel/QEMU is that the
> guest visible memory must always be 256MiB aligned due to PAPR mechanics that
> forces a memory block to be at least this size. Albeit I believe that there is
> no constraints of the memory this device is providing being counted as
> non-hotplugable, then in this case the alignment shouldn't be needed.

In Linux guests, virtio-mem adds whole memory blocks (e.g., aligned
256MB), but is able to expose only parts of a memory block dynamically
to Linux mm - essentially in 16MB on ppc64 IIRC.

E.g., on x86-64 (and soon arm64), we mostly add 128MB memory blocks, but
can operate on (currently) 4MB blocks (MAX_ORDER - 1) inside these
blocks. A little like memory ballooning ... but also quite different :)

So far the theory on ppc64. I have no prototype on ppc64, so we'll have
to see what's actually possible.

> But I digress. Thanks for the insights. I'll ping some people inside IBM and
> see if we have a more immediate use case for virtio-mem in Power. Perhaps
> we can do some sort of collaboration with your work.

Sure, I'll be happy to assist.


David / dhildenb

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