On Tue, Jan 23, 2024 at 2:28 PM Russell King (Oracle) <linux@xxxxxxxxxxxxxxx> wrote: > > On Mon, Jan 22, 2024 at 06:00:13PM +0000, Jonathan Cameron wrote: > > On Mon, 18 Dec 2023 21:35:16 +0100 > > "Rafael J. Wysocki" <rafael@xxxxxxxxxx> wrote: > > > > > On Wed, Dec 13, 2023 at 1:49 PM Russell King <rmk+kernel@xxxxxxxxxxxxxxx> wrote: > > > > > > > > From: James Morse <james.morse@xxxxxxx> > > > > > > > > The code behind ACPI_HOTPLUG_CPU allows a not-present CPU to become > > > > present. > > > > > > Right. > > > > > > > This isn't the only use of HOTPLUG_CPU. On arm64 and riscv > > > > CPUs can be taken offline as a power saving measure. > > > > > > But still there is the case in which a non-present CPU can become > > > present, isn't it there? > > > > Not yet defined by the architectures (and I'm assuming it probably never will be). > > > > The original proposal we took to ARM was to do exactly that - they pushed > > back hard on the basis there was no architecturally safe way to implement it. > > Too much of the ARM arch has to exist from the start of time. > > > > https://lore.kernel.org/linux-arm-kernel/cbaa6d68-6143-e010-5f3c-ec62f879ad95@xxxxxxx/ > > is one of the relevant threads of the kernel side of that discussion. > > > > Not to put specific words into the ARM architects mouths, but the > > short description is that there is currently no demand for working > > out how to make physical CPU hotplug possible, as such they will not > > provide an architecturally compliant way to do it for virtual CPU hotplug and > > another means is needed (which is why this series doesn't use the present bit > > for that purpose and we have the Online capable bit in MADT/GICC) > > > > It was a 'fun' dance of several years to get to that clarification. > > As another fun fact, the same is defined for x86, but I don't think > > anyone has used it yet (GICC for ARM has an online capable bit in the flags to > > enable this, which was remarkably similar to the online capable bit in the > > flags of the Local APIC entries as added fairly recently). > > > > > > > > > On arm64 an offline CPU may be disabled by firmware, preventing it from > > > > being brought back online, but it remains present throughout. > > > > > > > > Adding code to prevent user-space trying to online these disabled CPUs > > > > needs some additional terminology. > > > > > > > > Rename the Kconfig symbol CONFIG_ACPI_HOTPLUG_PRESENT_CPU to reflect > > > > that it makes possible CPUs present. > > > > > > Honestly, I don't think that this change is necessary or even useful. > > > > Whilst it's an attempt to avoid future confusion, the rename is > > not something I really care about so my advice to Russell is drop > > it unless you are attached to it! > > While I agree that it isn't a necessity, I don't fully agree that it > isn't useful. > > One of the issues will be that while Arm64 will support hotplug vCPU, > it won't be setting ACPI_HOTPLUG_CPU because it doesn't support > the present bit changing. So I can see why James decided to rename > it - because with Arm64's hotplug vCPU, the idea that ACPI_HOTPLUG_CPU > somehow enables hotplug CPU support is now no longer true. > > Keeping it as ACPI_HOTPLUG_CPU makes the code less obvious, because it > leads one to assume that it ought to be enabled for Arm64's > implementatinon, and that could well cause issues in the future if > people make the assumption that "ACPI_HOTPLUG_CPU" means hotplug CPU > is supported in ACPI. It doesn't anymore. On x86 there is no confusion AFAICS. It's always meant "as long as the platform supports it".
