linux-kernel - Re: [PATCH v3 2/2] LoongArch: Add paravirt support with vcpu_is

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Message-ID: <c9e3cd01-0c88-adc2-fdd1-ed85414c7550@loongson.cn>
Date: Thu, 18 Dec 2025 20:10:07 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>, WANG Xuerui <kernel@...0n.name>,
 Juergen Gross <jgross@...e.com>, Ajay Kaher <ajay.kaher@...adcom.com>,
 Alexey Makhalov <alexey.makhalov@...adcom.com>,
 Broadcom internal kernel review list
 <bcm-kernel-feedback-list@...adcom.com>, kvm@...r.kernel.org,
 loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
 virtualization@...ts.linux.dev, x86@...nel.org
Subject: Re: [PATCH v3 2/2] LoongArch: Add paravirt support with
 vcpu_is_preempted() in guest side



On 2025/12/6 下午9:04, Huacai Chen wrote:
> Hi, Bibo,
> 
> On Tue, Dec 2, 2025 at 10:48 AM Bibo Mao <maobibo@...ngson.cn> wrote:
>>
>> Function vcpu_is_preempted() is used to check whether vCPU is preempted
>> or not. Here add implementation with vcpu_is_preempted() when option
>> CONFIG_PARAVIRT is enabled.
>>
>> Signed-off-by: Bibo Mao <maobibo@...ngson.cn>
>> ---
>>   arch/loongarch/include/asm/qspinlock.h |  3 +++
>>   arch/loongarch/kernel/paravirt.c       | 23 ++++++++++++++++++++++-
>>   2 files changed, 25 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/loongarch/include/asm/qspinlock.h b/arch/loongarch/include/asm/qspinlock.h
>> index e76d3aa1e1eb..fa3eaf7e48f2 100644
>> --- a/arch/loongarch/include/asm/qspinlock.h
>> +++ b/arch/loongarch/include/asm/qspinlock.h
>> @@ -34,6 +34,9 @@ static inline bool virt_spin_lock(struct qspinlock *lock)
>>          return true;
>>   }
>>
>> +#define vcpu_is_preempted      vcpu_is_preempted
>> +bool vcpu_is_preempted(int cpu);
>> +
>>   #endif /* CONFIG_PARAVIRT */
>>
>>   #include <asm-generic/qspinlock.h>
>> diff --git a/arch/loongarch/kernel/paravirt.c b/arch/loongarch/kernel/paravirt.c
>> index b1b51f920b23..b61a93c6aec8 100644
>> --- a/arch/loongarch/kernel/paravirt.c
>> +++ b/arch/loongarch/kernel/paravirt.c
>> @@ -246,6 +246,7 @@ static void pv_disable_steal_time(void)
>>   }
>>
>>   #ifdef CONFIG_SMP
>> +static DEFINE_STATIC_KEY_FALSE(virt_preempt_key);
>>   static int pv_time_cpu_online(unsigned int cpu)
>>   {
>>          unsigned long flags;
>> @@ -267,6 +268,18 @@ static int pv_time_cpu_down_prepare(unsigned int cpu)
>>
>>          return 0;
>>   }
>> +
>> +bool notrace vcpu_is_preempted(int cpu)
>> +{
>> +       struct kvm_steal_time *src;
>> +
>> +       if (!static_branch_unlikely(&virt_preempt_key))
>> +               return false;
>> +
>> +       src = &per_cpu(steal_time, cpu);
>> +       return !!(src->preempted & KVM_VCPU_PREEMPTED);
>> +}
>> +EXPORT_SYMBOL(vcpu_is_preempted);
>>   #endif
>>
>>   static void pv_cpu_reboot(void *unused)
>> @@ -308,6 +321,9 @@ int __init pv_time_init(void)
>>                  pr_err("Failed to install cpu hotplug callbacks\n");
>>                  return r;
>>          }
>> +
>> +       if (kvm_para_has_feature(KVM_FEATURE_PREEMPT))
>> +               static_branch_enable(&virt_preempt_key);
>>   #endif
>>
>>          static_call_update(pv_steal_clock, paravt_steal_clock);
>> @@ -318,7 +334,12 @@ int __init pv_time_init(void)
>>                  static_key_slow_inc(&paravirt_steal_rq_enabled);
>>   #endif
>>
>> -       pr_info("Using paravirt steal-time\n");
>> +#ifdef CONFIG_SMP
> 
> Linux kernel is removing non-SMP step by step [1].
> https://kernelnewbies.org/Linux_6.17#Unconditionally_compile_task_scheduler_with_SMP_support
> 
> Though we cannot remove all "#ifdef CONFIG_SMP" at present, we can at
> least stop adding more.
> 
> So I prefer to make this whole patch out of CONFIG_SMP. But if you
> don't like this, you can at least move the virt_preempt_key
> declaration out of "#ifdef CONFIG_SMP", then the #ifdefs here can be
> removed.
Sorry, I just notice this mail.
Will move virt_preempt_key out of CONFIG_SMP in next version.

Regards
Bibo Mao
> 
> Huacai
> 
>> +       if (static_key_enabled(&virt_preempt_key))
>> +               pr_info("Using paravirt steal-time with preempt enabled\n");
>> +       else
>> +#endif
>> +               pr_info("Using paravirt steal-time with preempt disabled\n");
>>
>>          return 0;
>>   }
>> --
>> 2.39.3
>>
>>