From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v6 05/11] pvqspinlock, x86: Allow unfair spinlock in a
	PV guest
Date: Wed, 19 Mar 2014 18:08:07 +0100
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To: Waiman Long <waiman.long@hp.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>, Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>, kvm@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>, virtualization@lists.linux-foundation.org, Andi Kleen <andi@firstfloor.org>, "H. Peter Anvin" <hpa@zytor.com>, Michel Lespinasse <walken@google.com>, Alok Kataria <akataria@vmware.com>, linux-arch@vger.kernel.org, Gleb Natapov <gleb@redhat.com>, x86@kernel.org, Ingo Molnar <mingo@redhat.com>, xen-devel@lists.xenproject.org, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Rik van Riel <riel@redhat.com>, Arnd Bergmann <arnd@arndb.de>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, Scott J Norton <scott.norton@hp.com>, Steven Rostedt <rostedt@goodmis.org>, Chris Wright <chrisw@sous-sol.org>, Thomas Gleixner <tglx@linutronix.de>, Aswin Chandramouleeswaran <aswin@hp.com>, Chegu Vinod <chegu_vinod@hp.com>
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Il 19/03/2014 17:58, Waiman Long ha scritto:
>> Exactly.  What you want is boot_cpu_has(X86_FEATURE_HYPERVISOR).
>
> The unfair lock is to be enabled by boot time check, not just by the
> presence of a configuration macro during the build process in order to
> avoid using unfair lock on bare metal. Of course, Linux distros can
> modify this if that suits their need.

"boot_cpu_has" is a run-time check.  You can use it after setup_arch has 
called init_hypervisor_platform and kvm_guest_init.  Can you just just 
check if the PV path has been enabled and, if not, do a static_key_inc 
to enable the unfair path?

Paolo