On Thu, Sep 14, 2023 at 05:34:25PM +0200, Ard Biesheuvel wrote: > On Thu, 14 Sept 2023 at 16:55, Jonathan Cameron > <Jonathan.Cameron@xxxxxxxxxx> wrote: > > > > On Thu, 14 Sep 2023 09:57:44 +0200 > > Ard Biesheuvel <ardb@xxxxxxxxxx> wrote: > > > > > Hello James, > > > > > > On Wed, 13 Sept 2023 at 18:41, James Morse <james.morse@xxxxxxx> wrote: > > > > > > > > Add the new flag field to the MADT's GICC structure. > > > > > > > > 'Online Capable' indicates a disabled CPU can be enabled later. > > > > > > > > > > Why do we need a bit for this? What would be the point of describing > > > disabled CPUs that cannot be enabled (and are you are aware of > > > firmware doing this?). > > > > Enabled being not set is common at some similar ACPI tables at least. > > > > This is available in most ACPI tables to allow firmware to use 'nearly' > > static tables and just tweak the 'enabled' bit to say if the record should > > be ignored or not. Also _STA not present which is for same trick. > > If you are doing clever dynamic tables, then you can just not present > > the entry. > > > > With that existing use case in mind, need another bit to say this > > one might one day turn up. Note this is copied from x86 though no > > one seems to have implemented the kernel support for them yet. > > > > Note as per my other reply - this isn't a code first proposal. It's in the > > spec already (via a code first proposal last year I think). > > > > > > > > So why are we not able to assume that this new bit can always be treated as '1'? > > > > Given above, need the extra bit to size stuff to allow for the CPU showing up > > late. > > > > So does this mean that on x86, the CPU object is instantiated only > when the hardware level hotplug occurs? And before that, the object > does not exist at all? > > Because it seems to me that _STA, having both enabled and present > bits, could already describe what we need here, and arguably, a CPU > that is not both present and enabled should not be used by the OS. > This would leave room for representing off-line CPUs as present but > not enabled. > > Apologies if I am missing something obvious here - the whole rationale > behind this thing is rather confusing to me. Note that the bit is in the ACPI spec: https://uefi.org/specs/ACPI/6.5/05_ACPI_Software_Programming_Model.html#gicc-cpu-interface-flags The new bit has the same description as per the local-APIC equivalent: https://uefi.org/specs/ACPI/6.5/05_ACPI_Software_Programming_Model.html#local-apic-flags for a popular architecture that does have hot-pluggable physical CPUs ;) -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!
