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

[Eduardo Habkost]

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?

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