linux-kernel - Re: [PATCH v2 2/2] paravirt,xen: correct xen

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Message-ID: <4af731c9-16db-e386-7eb1-3d250dd6f4d5@redhat.com>
Date:   Wed, 6 Sep 2017 11:50:05 -0400
From:   Waiman Long <longman@...hat.com>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        virtualization@...ts.linux-foundation.org
Cc:     jeremy@...p.org, chrisw@...s-sol.org, akataria@...are.com,
        rusty@...tcorp.com.au, boris.ostrovsky@...cle.com, hpa@...or.com,
        tglx@...utronix.de, mingo@...hat.com, peterz@...radead.org
Subject: Re: [PATCH v2 2/2] paravirt,xen: correct xen_nopvspin case

On 09/06/2017 11:29 AM, Juergen Gross wrote:
> With the boot parameter "xen_nopvspin" specified a Xen guest should not
> make use of paravirt spinlocks, but behave as if running on bare
> metal. This is not true, however, as the qspinlock code will fall back
> to a test-and-set scheme when it is detecting a hypervisor.
>
> In order to avoid this disable the virt_spin_lock_key.
>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  arch/x86/xen/spinlock.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> index 25a7c4302ce7..e8ab80ad7a6f 100644
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -10,6 +10,7 @@
>  #include <linux/slab.h>
>  
>  #include <asm/paravirt.h>
> +#include <asm/qspinlock.h>
>  
>  #include <xen/interface/xen.h>
>  #include <xen/events.h>
> @@ -129,6 +130,7 @@ void __init xen_init_spinlocks(void)
>  
>  	if (!xen_pvspin) {
>  		printk(KERN_DEBUG "xen: PV spinlocks disabled\n");
> +		static_branch_disable(&virt_spin_lock_key);
>  		return;
>  	}
>  	printk(KERN_DEBUG "xen: PV spinlocks enabled\n");

Acked-by: Waiman Long <longman@...hat.com>