Re: [External] Re: [RFC] x86/kvm: Use native qspinlock by default when realtime hinted

[Konrad Rzeszutek Wilk]

On Wed, Jul 02, 2025 at 08:23:58PM +0800, Liangyan wrote:
> We test that unixbench spawn has big improvement in Intel 8582c 120-CPU
> guest vm if switch to qspinlock.

And ARM or AMD?

> 
> Command: ./Run -c 120 spawn
> 
> Use virt_spin_lock:
> System Benchmarks Partial Index   BASELINE       RESULT  INDEX
> Process Creation                     126.0      71878.4   5704.6
>                                                         ========
> System Benchmarks Index Score (Partial Only)              5704.6
> 
> 
> Use qspinlock:
> System Benchmarks Partial Index   BASELINE       RESULT    INDEX
> Process Creation                     126.0     173566.6  13775.1
>                                                         ========
> System Benchmarks Index Score (Partial Only              13775.1
> 
> 
> Regards,
> Liangyan
> 
> On 2025/7/2 16:19, Bibo Mao wrote:
> > 
> > 
> > On 2025/7/2 下午2:42, Liangyan wrote:
> > > When KVM_HINTS_REALTIME is set and KVM_FEATURE_PV_UNHALT is clear,
> > > currently guest will use virt_spin_lock.
> > > Since KVM_HINTS_REALTIME is set, use native qspinlock should be safe
> > > and have better performance than virt_spin_lock.
> > Just be curious, do you have actual data where native qspinlock has
> > better performance than virt_spin_lock()?
> > 
> > By my understanding, qspinlock is not friendly with VM. When lock is
> > released, it is acquired with one by one order in contending queue. If
> > the first vCPU in contending queue is preempted, the other vCPUs can not
> > get lock. On physical machine it is almost impossible that CPU
> > contending lock is preempted.
> > 
> > Regards
> > Bibo Mao
> > > 
> > > Signed-off-by: Liangyan <liangyan.peng@xxxxxxxxxxxxx>
> > > ---
> > >   arch/x86/kernel/kvm.c | 18 +++++++++---------
> > >   1 file changed, 9 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > > index 921c1c783bc1..9080544a4007 100644
> > > --- a/arch/x86/kernel/kvm.c
> > > +++ b/arch/x86/kernel/kvm.c
> > > @@ -1072,6 +1072,15 @@ static void kvm_wait(u8 *ptr, u8 val)
> > >    */
> > >   void __init kvm_spinlock_init(void)
> > >   {
> > > +    /*
> > > +     * Disable PV spinlocks and use native qspinlock when dedicated
> > > pCPUs
> > > +     * are available.
> > > +     */
> > > +    if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
> > > +        pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME
> > > hints\n");
> > > +        goto out;
> > > +    }
> > > +
> > >       /*
> > >        * In case host doesn't support KVM_FEATURE_PV_UNHALT there is
> > > still an
> > >        * advantage of keeping virt_spin_lock_key enabled:
> > > virt_spin_lock() is
> > > @@ -1082,15 +1091,6 @@ void __init kvm_spinlock_init(void)
> > >           return;
> > >       }
> > > -    /*
> > > -     * Disable PV spinlocks and use native qspinlock when dedicated
> > > pCPUs
> > > -     * are available.
> > > -     */
> > > -    if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
> > > -        pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME
> > > hints\n");
> > > -        goto out;
> > > -    }
> > > -
> > >       if (num_possible_cpus() == 1) {
> > >           pr_info("PV spinlocks disabled, single CPU\n");
> > >           goto out;
> > > 
> > 
> 
>