Re: [PATCH v2] x86/paravirt: Disable virt spinlock on bare metal

[Nikolay Borisov <nik.borisov@suse.com>]

On 26.05.24 г. 4:58 ч., Chen Yu wrote:
> The kernel can change spinlock behavior when running as a guest. But
> this guest-friendly behavior causes performance problems on bare metal.
> So there's a 'virt_spin_lock_key' static key to switch between the two
> modes.
> 
> The static key is always enabled by default (run in guest mode) and
> should be disabled for bare metal (and in some guests that want native
> behavior).
> 
> Performance drop is reported when running encode/decode workload and
> BenchSEE cache sub-workload.
> Bisect points to commit ce0a1b608bfc ("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
> decrease the performance on bare metal.
> 
> Fix this by disabling virt_spin_lock_key if it is on bare metal,
> regardless of CONFIG_PARAVIRT_SPINLOCKS.
> 

nit:

This bug wouldn't have happened if the key was defined FALSE by default 
and only enabled in the appropriate case. I think it makes more sense to 
invert the logic and have the key FALSE by default and only enable it 
iff the kernel is running under a hypervisor... At worst only the 
virtualization case would suffer if the lock is falsely not enabled.