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    SubjectRe: [PATCH 3/4] xen: move XEN_BALLOON_MEMORY_HOTPLUG to generic config
    On Wed, Aug 10, 2022 at 1:32 PM Oleksandr Tyshchenko
    <Oleksandr_Tyshchenko@epam.com> wrote:
    >
    >
    > On 10.08.22 08:07, Lukas Bulwahn wrote:
    >
    > Hello Lukas, all
    >
    > > While reviewing arch/x86/configs/xen.config, I noticed the following
    > > note in this file:
    > >
    > > '# depends on MEMORY_HOTPLUG, arm64 doesn't enable this yet,
    > > '# move to generic config if it ever does.
    > > CONFIG_XEN_BALLOON_MEMORY_HOTPLUG=y
    > >
    > > Commit 6c6685055a28 ("kconfig: add xenconfig defconfig helper") in June
    > > 2015 adds this note. Fortunately, commit 4ab215061554 ("arm64: Add memory
    > > hotplug support") from December 2018 adds the memory hotplug in arm64, so
    > > the precondition of this note above is now met.
    > >
    > > Move setting the config XEN_BALLOON_MEMORY_HOTPLUG into the generic
    > > xen.config now.
    >
    >
    > I might have missed something but I have never heard of anyone using
    > XEN_BALLOON_MEMORY_HOTPLUG on Arm.
    >
    > Looking into the code, I am afraid, CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
    > won't be functional on Arm without reworking resource allocation logic
    > in drivers/xen/ballon.c at least, since system "iomem_resource" cannot
    > be used on Arm to allocate unused memory region(s), please see
    > additional_memory_resource(). I might be wrong, but it feels to me that
    > here we would need a similar logic like in
    > drivers/xen/unpopulated-alloc.c which is functional on Arm since it uses
    > specific Xen resource (on x86 it is just an iomem_resource, but on Arm
    > it contains provided by the hypervisor extended regions).
    >
    > I am not aware of anyone working on it, so I wouldn't enable that
    > support on Arm by default until the code is updated.
    >

    Thanks for the in-depth investigation and explanation. For now, let us
    drop this patch here and keep the configs as they are.

    Lukas

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