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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Mon, 23 Jul 2018 11:31:13 +0800
Message-ID: <CANRm+CyHfvBKVKNk+pKF+s6+1seZ8c+NyjHuepT2Mf+4Cdd-2w@mail.gmail.com>
Subject: Re: [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU
To:     Waiman Long <longman@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim Krcmar <rkrcmar@redhat.com>
Cc:     Davidlohr Bueso <dave@stgolabs.net>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        xen-devel <xen-devel@lists.xenproject.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
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Sender: linux-kernel-owner@vger.kernel.org
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On Fri, 20 Jul 2018 at 06:03, Waiman Long <longman@redhat.com> wrote:
>
> On 07/19/2018 05:54 PM, Davidlohr Bueso wrote:
> > On Thu, 19 Jul 2018, Waiman Long wrote:
> >
> >> On a VM with only 1 vCPU, the locking fast paths will always be
> >> successful. In this case, there is no need to use the the PV qspinlock
> >> code which has higher overhead on the unlock side than the native
> >> qspinlock code.
> >>
> >> The xen_pvspin veriable is also turned off in this 1 vCPU case to
> >> eliminate unneeded pvqspinlock initialization in xen_init_lock_cpu()
> >> which is run after xen_init_spinlocks().
> >
> > Wouldn't kvm also want this?
> >
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index a37bda38d205..95aceb692010 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -457,7 +457,8 @@ static void __init sev_map_percpu_data(void)
> > static void __init kvm_smp_prepare_cpus(unsigned int max_cpus)
> > {
> >     native_smp_prepare_cpus(max_cpus);
> > -    if (kvm_para_has_hint(KVM_HINTS_REALTIME))
> > +    if (num_possible_cpus() == 1 ||
> > +        kvm_para_has_hint(KVM_HINTS_REALTIME))
> >         static_branch_disable(&virt_spin_lock_key);
> > }
>
> That doesn't really matter as the slowpath will never get executed in
> the 1 vCPU case.

So this is not needed in kvm tree?
https://git.kernel.org/pub/scm/virt/kvm/kvm.git/commit/?h=queue&id=3a792199004ec335346cc607d62600a399a7ee02

Regards,
Wanpeng Li