Comment 0 for bug 1157757

When testing I found that when I boot a Xen HVM guest on newer Intel based systems (maybe starting with Sandy Bridge) none of the additional VCPUs come online:

cpu 1 spinlock event irq 70
Booting Node 0, Processors #1
CPU1: Stuck ??

This does not happen on my AMD Opteron host and neither on a box with an old i7 (one of the first ones that came out). This started with kernels between 3.4 and 3.5-rc1, so Quantal and onwards. I was able to limit the range via bisect (unfortunately within that range the kernel does not build):

323f90a xen-acpi-processor: Add missing #include <xen/xen.h>
2ee93ab acpi, bgrd: Add missing <linux/io.h> to drivers/acpi/bgrt.c
638d957 x86, realmode: Change EFER to a single u64 field
1371270 x86, realmode: Move kernel/realmode.c to realmode/init.c
51edbe6 x86, realmode: Move not-common bits out of trampoline_common.S
7960387 x86, realmode: Mask out EFER.LMA when saving trampoline EFER
34d0b02 x86, realmode: Fix no cache bits test in reboot_32.S
0f6f11eb x86, realmode: Make sure all generated files are listed in targets
c5403ae x86, realmode: build fix: remove duplicate build
cda846f x86, realmode: read cr4 and EFER from kernel for 64-bit trampoline
bf8b88e x86, realmode: fixes compilation issue in tboot.c
f2604c1 x86, realmode: move relocs from scripts/ to arch/x86/tools
f37240f x86, realmode: header for trampoline code
c484547 x86, realmode: flattened rm hierachy
b429dbf x86, realmode: don't copy real_mode_header
8e029fc x86, realmode: fix 64-bit wakeup sequence
6feb592 x86, realmode: Fix always-zero test in reboot_32.S
be60828 x86, realmode: Move trampoline_*.S early in the link order
e5684ec x86, realmode: Replace open-coded ljmpw with a macro
968ff9e x86, realmode: Remove indirect jumps in trampoline_32 and wakeup_asm
056a43a x86, realmode: Remove indirect jumps in trampoline_64.S
f7436a9 x86, realmode: Align .data section in trampoline_32.S

Not sure why this only affects certain Intel CPUs, maybe some VMX feature that has some side-effect on the changes in the realmode code.