On Mon, Jul 23, 2012 at 06:40:51PM +0000, Blue Swirl wrote: > On Fri, Jul 20, 2012 at 8:00 PM, Eduardo Habkost <ehabk...@redhat.com> wrote: > > Hi, > > > > While working at the CPU index vs APIC ID changes, I stumbled upon > > another not-very-well-defined interface between SeaBIOS and QEMU, and I > > would like to clarify the semantics and constraints of some FW_CFG > > entries. > > > > First, the facts/assumptions: > > > > - There's no concept of "CPU index" or "CPU identifier" that SeaBIOS and > > QEMU agree upon, except for the APIC ID. All SeaBIOS can really see > > are the CPU APIC IDs, on boot or on CPU hotplug. > > - The APIC ID is already a perfectly good CPU identifier, that is > > present on bare metal hardware too. > > - Adding a new kind of "CPU identifier" in addition to the APIC ID > > would just make things more complex. > > - The only problem with APIC IDs is that they may not be contiguous. > > > > That said, I would like to clarify the meaning of: > > > > - FW_CFG_MAX_CPUS > > > > What are the basic semantics and expectations about FW_CFG_MAX_CPUS? > > FYI: This originates from Sparc and PPC, it says how many SMP CPUs > there are in the system. There we don't have (at least now) any CPU > IDs and of course no APIC.
Aren't you describing FW_CFG_NB_CPUS? If not, what's the difference between FS_CFG_NB_CPUS and FW_CFG_MAX_CPUS on those architectures? Until now, the only purpose I see for max_cpus/FW_CFG_MAX_CPUS is to allow CPU hotplug. I don't know if there are other use cases where max_cpus/FW_CFG_MAX_CPUS is useful. > > But I have no idea what x86 should use. As a general rule, what would > happen on a real machine should be emulated, but QEMU can also assist > BIOS (for example to skip some complex HW probes). Right now I am divided between two approaches: - In case FW_CFG_MAX_CPUS' only purpose is to allow CPU hotplug, make it really mean "upper limit to APIC ID values" in x86; - Otherwise, I am inclined to add a FW_CFG_MAX_APIC_ID entry to x86, so the firmware can (optionally) choose appropriate sizes for its internal APIC-ID-based data structures. > > > Considering that the APIC IDs may not be contiguous, is it supposed to > > be: > > > > a) the maximum number of CPUs that will be ever online, doesn't matter > > their APIC IDs, or > > b) a value so that every CPU has APIC ID < MAX_CPUS. > > > > A practical example: suppose we have a machine with 18 CPUs with the > > following APIC IDs: 0x00, 0x01, 0x02, 0x04, 0x05, 0x06, 0x08, 0x09, > > 0x0a, 0x10, 0x11, 0x12, 0x14, 0x15, 0x16, 0x18, 0x19, 0x1a. > > > > (That's the expected result for a machine with 2 sockets, 3 cores per > > socket, 3 threads per core.) > > > > In that case, should FW_CFG_MAX_CPUS be: a) 18, or b) 27 (0x1b)? > > > > If it should be 18, it will require additional work on SeaBIOS to make: > > - CPU hotplug work > > - SRAT/MADT/SSDT tables be built with Processor ID != APIC ID > > - SRAT/MADT/SSDT tables be kept stable if the system is hibernated and > > resumed after a CPU is hot-plugged. > > > > (Probably in that case I would suggest introducing a FW_CFG_MAX_APIC_ID > > entry, so that SeaBIOS can still build the ACPI tables more easily). > > > > > > - FW_CFG_NUMA > > > > The first problem with FW_CFG_NUMA is that it depends on FW_CFG_MAX_CPUS > > (so it inherits the same questions above). The second is that > > FW_CFG_NUMA is a CPU-based table, but there's nothing SeaBIOS can use to > > know what CPUs FW_CFG_NUMA is refering to, except for the APIC IDs. So, > > should FW_CFG_NUMA be indexed by APIC IDs? > > > > > > - My proposal: > > > > My proposal is to try to keep things simple, and just use the following > > rule: > > > > - Never have a CPU with APIC ID > FW_CFG_MAX_CPUS. > > > > This way: > > - The SeaBIOS ACPI code can be kept simple. > > - The current CPU hotplug interface can work as-is (up to 256 CPUs), > > based on APIC IDs. > > - The current FW_CFG_NUMA interface can work as-is, indexed by APIC IDs. > > - The ACPI tables can be easily kept stable between hibernate and > > resume, after CPU hotplug. > > > > This is the direction I am trying to go, and I am sending this just to > > make sure nobody is against it, and to not surprise anybody when I send > > a QEMU patch to make FW_CFG_MAX_CPUS be based on APIC IDs. > > > > > > My second proposal would be to introduce a FW_CFG_MAX_APIC_ID entry, so > > the SeaBIOS ACPI code can be kept simple. > > > > My third proposal would be to introduce a FW_CFG CPU Index => APIC ID > > table, but I really wouldn't like to introduce a new type of CPU > > identifier to be used between QEMU and SeaBIOS, when the APIC ID is a > > perfectly good unique CPU identifier that already exists in bare metal > > hardware. > > > > -- > > Eduardo > > > -- Eduardo
