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Date:   Sun, 22 Jul 2018 21:42:57 -0700
From:   Davidlohr Bueso <dave@stgolabs.net>
To:     Wanpeng Li <kernellwp@gmail.com>
Cc:     Waiman Long <longman@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim Krcmar <rkrcmar@redhat.com>,
        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>
Subject: Re: [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU
Message-ID: <20180723044257.m7pjrnp7jjqggqij@linux-r8p5>
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On Mon, 23 Jul 2018, Wanpeng Li wrote:

>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

s/veriable
  variable

>> >> 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.

How does this differ then from xen, then? I mean, same principle applies.

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

Hmm I would think that my patch would be more appropiate as it actually does
what the comment says.

Thanks,
Davidlohr