linux-kernel - Re: [PATCH v3] x86/paravirt: Disable virt spinlock on bare metal

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Date: Tue, 25 Jun 2024 18:08:12 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
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>, x86@...nel.org,
 "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>, Chen Yu <yu.chen.surf@...il.com>,
 Qiuxu Zhuo <qiuxu.zhuo@...el.com>, Prem Nath Dey <prem.nath.dey@...el.com>,
 Xiaoping Zhou <xiaoping.zhou@...el.com>
Subject: Re: [PATCH v3] x86/paravirt: Disable virt spinlock on bare metal



On 25.06.24 г. 17:50 ч., Chen Yu wrote:
> On 2024-06-25 at 16:42:11 +0300, Nikolay Borisov wrote:
>>
>>
>> On 25.06.24 г. 15:54 ч., Chen Yu wrote:
>>> The kernel can change spinlock behavior when running as a guest. But
>>> this guest-friendly behavior causes performance problems on bare metal.
>>> So there's a 'virt_spin_lock_key' static key to switch between the two
>>> modes.
>>>
>>> The static key is always enabled by default (run in guest mode) and
>>> should be disabled for bare metal (and in some guests that want native
>>> behavior).
>>>
>>> Performance drop is reported when running encode/decode workload and
>>> BenchSEE cache sub-workload.
>>> Bisect points to commit ce0a1b608bfc ("x86/paravirt: Silence unused
>>> native_pv_lock_init() function warning"). When CONFIG_PARAVIRT_SPINLOCKS
>>> is disabled the virt_spin_lock_key is incorrectly set to true on bare
>>> metal. The qspinlock degenerates to test-and-set spinlock, which
>>> decrease the performance on bare metal.
>>>
>>> Set the default value of virt_spin_lock_key to false. If booting in a VM,
>>> enable this key. Later during the VM initialization, if other
>>> high-efficient spinlock is preferred(paravirt-spinlock eg), the
>>> virt_spin_lock_key is disabled accordingly. The relation is described as
>>> below:
>>>
>>> X86_FEATURE_HYPERVISOR         Y    Y    Y     N
>>> CONFIG_PARAVIRT_SPINLOCKS      Y    Y    N     Y/N
>>> PV spinlock                    Y    N    N     Y/N
>>>
>>> virt_spin_lock_key             N    N    Y     N
>>>
>>> -DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
>>> +DEFINE_STATIC_KEY_FALSE(virt_spin_lock_key);
>>>    void __init native_pv_lock_init(void)
>>>    {
>>> -	if (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) &&
>>
>> Actually now shouldn't the CONFIG_PARAVIRT_SPINLOCKS check be retained?
>> Otherwise we'll have the virtspinlock enabled even if we are a guest but
>> CONFIG_PARAVIRT_SPINLOCKS is disabled, no ?
>>
> 
> It seems to be the expected behavior? If CONFIG_PARAVIRT_SPINLOCKS is disabled,
> should the virt_spin_lock_key be enabled in the guest?

No, but if it's disabled and we are under a hypervisor shouldn't the 
virt spinlock be kept disabled? As it stands now everytime we are under 
a hypervisor the virt spinlock is enabled irrespective of the 
PARAVIRT_SPINLOCK config state.

> The previous behavior before commit ce0a1b608bfc ("x86/paravirt: Silence unused
> native_pv_lock_init() function warning"): kvm_spinlock_init() is NULL if
> CONFIG_PARAVIRT_SPINLOCKS is disabled, and static_branch_disable(&virt_spin_lock_key)
> can not be invoked, so the virt_spin_lock_key keeps enabled.
> 
> thanks,
> Chenyu
>