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

From: maobibo
Date: Tue Jul 30 2024 - 05:39:22 EST




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