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

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Message-ID: <CAAhV-H6D_XxGTgWjzO26JtBcNeaouqrzH1wTaCn7xK3HGtZ55w@mail.gmail.com>
Date: Sat, 6 Dec 2025 21:04:28 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Bibo Mao <maobibo@...ngson.cn>
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

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.

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
>
>