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Peter Anvin" , "kernel-janitors@vger.kernel.org" , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM, RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 10, 2022 at 1:32 PM Oleksandr Tyshchenko 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