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

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Message-ID: <76216f5d-ade4-bd5b-980e-d616e77370a5@loongson.cn>
Date: Tue, 30 Jul 2024 17:39:06 +0800
From: maobibo <maobibo@...ngson.cn>
To: Chen Yu <yu.c.chen@...el.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>,
 Arnd Bergmann <arnd@...db.de>, virtualization@...ts.linux.dev,
 linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>,
 Nikolay Borisov <nik.borisov@...e.com>, Qiuxu Zhuo <qiuxu.zhuo@...el.com>,
 Prem Nath Dey <prem.nath.dey@...el.com>,
 Xiaoping Zhou <xiaoping.zhou@...el.com>,
 Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v4] x86/paravirt: Disable virt spinlock on bare metal

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
>