Re: [Qemu-devel] SeaBIOS, FW_CFG_NUMA, and FW_CFG_MAX_CPUS

[Blue Swirl]

On Mon, Jul 23, 2012 at 7:09 PM, Eduardo Habkost <address@hidden> 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 <address@hidden> 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