On Mon, Jul 23, 2012 at 7:09 PM, Eduardo Habkost <ehabk...@redhat.com> wrote: > 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?
Yes, sorry. There's no difference. > > 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. One integer does not tell very much. > >> >> > 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
