linux

The x86_capability array in cpuinfo_x86 is of type u32 and thus is
naturally aligned to 4 bytes. But, set_bit() and clear_bit() require the
array to be aligned to size of unsigned long (i.e. 8 bytes on 64-bit
systems).

The array pointer is handed into atomic bit operations. If the access is
not aligned to unsigned long then the atomic bit operations can end up
crossing a cache line boundary, which causes the CPU to do a full bus lock
as it can't lock both cache lines at once. The bus lock operation is heavy
weight and can cause severe performance degradation.

The upcoming #AC split lock detection mechanism will issue warnings for
this kind of access.

Force the alignment of the array to unsigned long. This avoids the massive
code changes which would be required when converting the array data type to
unsigned long.

[ tglx: Rewrote changelog so it contains information WHY this is required ]

Suggested-by: David Laight <David.Laight@aculab.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190916223958.27048-4-tony.luck@intel.com