Re: [PATCH v4] x86/paravirt: Disable virt spinlock on bare metal

[maobibo <maobibo@loongson.cn>]

On 2024/7/30 下午4:46, Chen Yu wrote:
> Hi Bibo,
> 
> On 2024-07-30 at 09:21:45 +0800, maobibo wrote:
>> Chenyu,
>>
>> Sorry to bother you, I am porting pv spinlock to LoongArch platform, I do
>> not know the history about function virt_spin_lock().
>>
>> When CONFIG_PARAVIRT_SPINLOCKS is enabled, there is pv_enabled() before
>> virt_spin_lock(), it seems that virt_spin_lock is never called in this
>> condition.
> 
> Right, if CONFIG_PARAVIRT_SPINLOCKS is enabled, virt_spin_lock() will not be
> checked at all and go directly to pv_queue lock section.
> 
>>          if (pv_enabled())
>>                  goto pv_queue;
>>
>>          if (virt_spin_lock(lock))
>>                  return;
>>
>> When CONFIG_PARAVIRT_SPINLOCKS is disabled, there is no pv qspinlock
>> compiled, so what is the usage about function virt_spin_lock()?
>> Is it to switch spinlock method from test-set spinlock to qspinlock?
>> why is there such requirement for spinlock switching method?
> 
> Firstly, according to the commit 2aa79af64263 ("locking/qspinlock: Revert
> to test-and-set on hypervisors"), virt_spin_lock() was originally introduced
> to avoid the over-preemption of native qspinlock(when paravirt-spinlock is
> disabled).
Chenyu,

Thanks for the information. It seems that test-and-set spinlock is 
suggested rather than qspinlock on VM when paravirt-spinlock is disabled.

Regards
Bibo Mao
> 
> Secondly, there is user requirement to use native qspinlock over test-set lock
> in the VM. For example, nopvspin parameter is parsed in xen_init_spinlocks().
> commit 9043442b43b1 ("locking/paravirt: Use new static key for controlling call
> of virt_spin_lock()") was introduced to adjust the key from user boot up commandline.
> so the key switches between test-set and the native qspinlock. I've Cced
> Juergen and Peter in case I misunderstood.
> 
> thanks,
> Chenyu
>