Chris reported that a Dell PowerEdge T340 system stopped to boot when upgrading
to a kernel which contains the parallel hotplug changes.  Disabling parallel
hotplug on the kernel command line makes it boot again.

It turns out that the Dell BIOS has x2APIC enabled and the boot CPU comes up in
X2APIC mode, but the APs come up inconsistently in xAPIC mode.

Parallel hotplug requires that the upcoming CPU reads out its APIC ID from the
local APIC in order to map it to the Linux CPU number.

In this particular case the readout on the APs uses the MMIO mapped registers
because the BIOS failed to enable x2APIC mode. That readout results in a page
fault because the kernel does not have the APIC MMIO space mapped when X2APIC
mode was enabled by the BIOS on the boot CPU and the kernel switched to X2APIC
mode early. That page fault can't be handled on the upcoming CPU that early and
results in a silent boot failure.

If parallel hotplug is disabled the system boots because in that case the APIC
ID read is not required as the Linux CPU number is provided to the AP in the
smpboot control word. When the kernel uses x2APIC mode then the APs are
switched to x2APIC mode too slightly later in the bringup process, but there is
no reason to do it that late.

Cure the BIOS bogosity by checking in the parallel bootup path whether the
kernel uses x2APIC mode and if so switching over the APs to x2APIC mode before
the APIC ID readout.

Fixes: 0c7ffa32 ("x86/smpboot/64: Implement arch_cpuhp_init_parallel_bringup() and enable it")
Reported-by: Chris Lindee <chris.lindee@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Tested-by: Chris Lindee <chris.lindee@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/CA%2B2tU59853R49EaU_tyvOZuOTDdcU0RshGyydccp9R1NX9bEeQ@mail.gmail.com