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

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Date: Tue, 25 Jun 2024 22:50:11 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Nikolay Borisov <nik.borisov@...e.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 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?
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