The kernel can change spinlock behavior when running as a guest. But this
guest-friendly behavior causes performance problems on bare metal.

The kernel uses a static key to switch between the two modes.

In theory, the static key is enabled by default (run in guest mode) and
should be disabled for bare metal (and in some guests that want native
behavior or paravirt spinlock).

A performance drop is reported when running encode/decode workload and
BenchSEE cache sub-workload.

Bisect points to commit ce0a1b60 ("x86/paravirt: Silence unused
native_pv_lock_init() function warning"). When CONFIG_PARAVIRT_SPINLOCKS is
disabled the virt_spin_lock_key is incorrectly set to true on bare
metal. The qspinlock degenerates to test-and-set spinlock, which decreases
the performance on bare metal.

Set the default value of virt_spin_lock_key to false. If booting in a VM,
enable this key. Later during the VM initialization, if other
high-efficient spinlock is preferred (e.g. paravirt-spinlock), or the user
wants the native qspinlock (via nopvspin boot commandline), the
virt_spin_lock_key is disabled accordingly.

This results in the following decision matrix:

X86_FEATURE_HYPERVISOR         Y    Y       Y     N
CONFIG_PARAVIRT_SPINLOCKS      Y    Y       N     Y/N
PV spinlock                    Y    N       N     Y/N

virt_spin_lock_key             N    Y/N     Y     N

Fixes: ce0a1b60 ("x86/paravirt: Silence unused native_pv_lock_init() function warning")
Reported-by: Prem Nath Dey <prem.nath.dey@intel.com>
Reported-by: Xiaoping Zhou <xiaoping.zhou@intel.com>
Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Suggested-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Suggested-by: Nikolay Borisov <nik.borisov@suse.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20240806112207.29792-1-yu.c.chen@intel.com