Re: [PATCH 3/4] xen: move XEN_BALLOON_MEMORY_HOTPLUG to generic config

From: Lukas Bulwahn <lukas.bulwahn@gmail.com>
To: Oleksandr Tyshchenko <Oleksandr_Tyshchenko@epam.com>
Cc: Juergen Gross <jgross@suse.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"H . Peter Anvin" <hpa@zytor.com>,
	"kernel-janitors@vger.kernel.org"
	<kernel-janitors@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/4] xen: move XEN_BALLOON_MEMORY_HOTPLUG to generic config
Date: Wed, 17 Aug 2022 06:26:26 +0200	[thread overview]
Message-ID: <CAKXUXMx4fBTN5e3aO9Hy2j3==3QDr85bknLxjyGLoT0geUq=jQ@mail.gmail.com> (raw)
In-Reply-To: <081fa42d-8587-a69e-5e1c-672152ac8c3e@epam.com>

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