Re: [PATCH v3] x86/paravirt: Disable virt spinlock on bare metal

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

On 25.06.24 г. 17:50 ч., Chen Yu wrote:
> On 2024-06-25 at 16:42:11 +0300, Nikolay Borisov wrote:
>>
>>
>> On 25.06.24 г. 15:54 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.
>>>
>>> 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(paravirt-spinlock eg), the
>>> virt_spin_lock_key is disabled accordingly. The relation is described as
>>> below:
>>>
>>> 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    N    Y     N
>>>
>>> -DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
>>> +DEFINE_STATIC_KEY_FALSE(virt_spin_lock_key);
>>>    void __init native_pv_lock_init(void)
>>>    {
>>> -	if (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) &&
>>
>> Actually now shouldn't the CONFIG_PARAVIRT_SPINLOCKS check be retained?
>> Otherwise we'll have the virtspinlock enabled even if we are a guest but
>> CONFIG_PARAVIRT_SPINLOCKS is disabled, no ?
>>
> 
> It seems to be the expected behavior? If CONFIG_PARAVIRT_SPINLOCKS is disabled,
> should the virt_spin_lock_key be enabled in the guest?

No, but if it's disabled and we are under a hypervisor shouldn't the 
virt spinlock be kept disabled? As it stands now everytime we are under 
a hypervisor the virt spinlock is enabled irrespective of the 
PARAVIRT_SPINLOCK config state.

> The previous behavior before commit ce0a1b608bfc ("x86/paravirt: Silence unused
> native_pv_lock_init() function warning"): kvm_spinlock_init() is NULL if
> CONFIG_PARAVIRT_SPINLOCKS is disabled, and static_branch_disable(&virt_spin_lock_key)
> can not be invoked, so the virt_spin_lock_key keeps enabled.
> 
> thanks,
> Chenyu
>