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

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Date: Wed, 26 Jun 2024 07:38:28 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Chen Yu <yu.c.chen@...el.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>
Cc: 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. 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
> 
> 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>
> Suggested-by: Nikolay Borisov <nik.borisov@...e.com>
> Reported-by: Prem Nath Dey <prem.nath.dey@...el.com>
> Reported-by: Xiaoping Zhou <xiaoping.zhou@...el.com>
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>

Reviewed-by: Nikolay Borisov <nik.borisov@...e.com>