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DescriptionChris von Recklinghausen
2021-07-02 15:34:53 UTC
Description of problem:
The decision has been made that RHEL9 on aarch64 will have 4K pages. This is the bug for the kernel change.
Version-Release number of selected component (if applicable):
How reproducible:
Steps to Reproduce:
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Actual results:
Expected results:
Additional info:
The description is very sparse - could you please elaborate on the reasons for changing the page size?
Also, is virtio-balloon now working properly with KVM guests that have a different page size? (we had some problems there in the past, see https://bugzilla.redhat.com/show_bug.cgi?id=1324092 )
Comment 3David Hildenbrand
2021-07-07 10:57:45 UTC
(In reply to Thomas Huth from comment #1)
> The description is very sparse - could you please elaborate on the reasons
> for changing the page size?
>
> Also, is virtio-balloon now working properly with KVM guests that have a
> different page size? (we had some problems there in the past, see
> https://bugzilla.redhat.com/show_bug.cgi?id=1324092 )
All distributions except RHEL already switched to 4k. So we're already facing host=64k guest=4k on arm64 I guess.
virtio-balloon itself works on 4k "virtual" pages.
When the guest is 64k and the host is 4k, it works by communicating 16 virtual 4k pages to the host and "works properly". When the guest is 4k and the host is 64k, it only works if the guest reports multiple contiguous 4k pages that for a 64k page -- which is often the case, but not always. The host will discard a whole 64k page once it collected all 4k pages. But it's more an optimization we added for ppc64, and not a "working properly".
Anyhow, the future is 4k and e.g., virtio-mem will benefit heavily from that; if virtio-balloon is suboptimal for now, I don't think it's a show stopper. It was never designed to support differing page sizes, unfortunately, and we've been living with this fact on ppc64 already.