linux-kernel - Re: [PATCH v2] x86/paravirt: Disable virt spinlock on bare metal

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Message-ID: <c83127de-b59a-4475-8d20-e3582ee2e6f8@suse.com>
Date: Tue, 18 Jun 2024 11:24:42 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Chen Yu <yu.c.chen@...el.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Juergen Gross <jgross@...e.com>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Ajay Kaher <ajay.kaher@...adcom.com>
Cc: x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 Arnd Bergmann <arnd@...db.de>, virtualization@...ts.linux.dev,
 linux-kernel@...r.kernel.org, Qiuxu Zhuo <qiuxu.zhuo@...el.com>,
 Prem Nath Dey <prem.nath.dey@...el.com>,
 Xiaoping Zhou <xiaoping.zhou@...el.com>
Subject: Re: [PATCH v2] x86/paravirt: Disable virt spinlock on bare metal

On 26.05.24 г. 4:58 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.
> 
> Fix this by disabling virt_spin_lock_key if it is on bare metal,
> regardless of CONFIG_PARAVIRT_SPINLOCKS.
> 

nit:

This bug wouldn't have happened if the key was defined FALSE by default 
and only enabled in the appropriate case. I think it makes more sense to 
invert the logic and have the key FALSE by default and only enable it 
iff the kernel is running under a hypervisor... At worst only the 
virtualization case would suffer if the lock is falsely not enabled.