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

[Chen Yu <yu.c.chen@intel.com>]

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).

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