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

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Message-ID: <ZnDnEHV+Nxj0OpVs@chenyu5-mobl2>
Date: Tue, 18 Jun 2024 09:46:56 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: 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

Hello,

On 2024-05-26 at 09:58:47 +0800, 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.
> 
> Fixes: ce0a1b608bfc ("x86/paravirt: Silence unused native_pv_lock_init() function warning")
> Suggested-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Suggested-by: Qiuxu Zhuo <qiuxu.zhuo@...el.com>
> Reported-by: Prem Nath Dey <prem.nath.dey@...el.com>
> Reported-by: Xiaoping Zhou <xiaoping.zhou@...el.com>
> Reviewed-by: Juergen Gross <jgross@...e.com>
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>

A gentle ping on this patch :)

thanks,
Chenyu