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

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Message-ID: <dc4d14c5-1f04-47d7-b314-e4db62f57665@google.com>
Date: Fri, 4 Jul 2025 14:12:04 +0800
From: Liangyan <liangyan.peng@...edance.com>
To: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc: Bibo Mao <maobibo@...ngson.cn>, pbonzini@...hat.com, vkuznets@...hat.com,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, hpa@...or.com, wanpengli@...cent.com,
 linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [External] Re: [PATCH RFC] x86/kvm: Use native qspinlock by
 default when realtime hinted


Find one AMD guest(AMD EPYC 9Y24 128-vCPU) to test, it seems about 9% 
improvement.

Command: ./Run -c 128 spawn

With virt spin lock:
System Benchmarks Partial Index   BASELINE       RESULT    INDEX
Process Creation                      126.0     120449.8   9559.5
                                                           ========
System Benchmarks Index Score (Partial Only                9559.5


With qspinlock:
System Benchmarks Partial Index   BASELINE       RESULT    INDEX
Process Creation                      126.0     131566.8  10441.8
                                                           ========
System Benchmarks Index Score (Partial Only)              10441.8



Regards,
Liangyan


On 2025/7/3 00:26, Konrad Rzeszutek Wilk wrote:
> 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@...edance.com>
>>>> ---
>>>>    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;
>>>>
>>>
>>
>>