Re: [edk2-devel] [PATCH 0/1] OvmfPkg/Bhyve: QemuFwCfg support

["Gerd Hoffmann" <kraxel@redhat.com>]

On Tue, Mar 29, 2022 at 08:54:36AM +0200, Corvin Köhne wrote:
> Hi,
> 
> I'm going to add QemuFwCfg support to bhyve. See
> https://reviews.freebsd.org/D31578. Therefore, this patch for OVMF is
> neccessary to work properly.
> 
> There's one open point on that patch and hopefully one of you has more
> insights. Qemu has an item called FW_CFG_MAX_CPUS. It looks very similar to
> what we need here, but I'm not sure if we can use it safely as Qemu has a
> comment about it:
> 
> https://github.com/qemu/qemu/blob/0021c4765a6b83e5b09409b75d50c6caaa6971b9/hw/i386/fw_cfg.c#L110-L121
> 
>     /* FW_CFG_MAX_CPUS is a bit confusing/problematic on x86:
>      *
>      * For machine types prior to 1.8, SeaBIOS needs FW_CFG_MAX_CPUS for
>      * building MPTable, ACPI MADT, ACPI CPU hotplug and ACPI SRAT table,
>      * that tables are based on xAPIC ID and QEMU<->SeaBIOS interface
>      * for CPU hotplug also uses APIC ID and not "CPU index".
>      * This means that FW_CFG_MAX_CPUS is not the "maximum number of CPUs",
>      * but the "limit to the APIC ID values SeaBIOS may see".
>      *
>      * So for compatibility reasons with old BIOSes we are stuck with
>      * "etc/max-cpus" actually being apic_id_limit
>      */

There is FW_CFG_NB_CPUS + FW_CFG_MAX_CPUS.  ovmf uses different names,
see OvmfPkg/Include/IndustryStandard/QemuFwCfg.h

PlatformPei for qemu uses QemuFwCfgItemSmpCpuCount aka FW_CFG_NB_CPUS,
which is the number of cpus which are online.

I think FW_CFG_MAX_CPUS is basically unused these days.  It played a
role back when seabios created the acpi tables for cpu hotplug as
described in the comment above.  In qemu 2.0 & newer the acpi tables are
generated by qemu instead.  The firmware just downloads them from fw_cfg
and installs them for the OS, it doesn't need to know virtual machine
configuration details any more.

HTH,
  Gerd