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
