linux-kernel - Re: [PATCH v2 2/3] LoongArch: Add paravirt support with vcpu_is

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Message-ID: <CAAhV-H6TWCs-tFf5HCOt9cAY01J-nirzVmu1GEAYpP=1LWznPA@mail.gmail.com>
Date: Mon, 24 Nov 2025 17:03:51 +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 v2 2/3] LoongArch: Add paravirt support with
 vcpu_is_preempted() in guest side

On Mon, Nov 24, 2025 at 4:35 PM Bibo Mao <maobibo@...ngson.cn> wrote:
>
>
>
> On 2025/11/24 下午4:03, Huacai Chen wrote:
> > On Mon, Nov 24, 2025 at 3:50 PM Bibo Mao <maobibo@...ngson.cn> wrote:
> >>
> >>
> >>
> >> On 2025/11/24 下午3:13, Huacai Chen wrote:
> >>> On Mon, Nov 24, 2025 at 3:03 PM Bibo Mao <maobibo@...ngson.cn> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 2025/11/24 下午2:33, Huacai Chen wrote:
> >>>>> Hi, Bibo,
> >>>>>
> >>>>> On Mon, Nov 24, 2025 at 11:54 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 |  5 +++++
> >>>>>>     arch/loongarch/kernel/paravirt.c       | 16 ++++++++++++++++
> >>>>>>     2 files changed, 21 insertions(+)
> >>>>>>
> >>>>>> diff --git a/arch/loongarch/include/asm/qspinlock.h b/arch/loongarch/include/asm/qspinlock.h
> >>>>>> index e76d3aa1e1eb..9a5b7ba1f4cb 100644
> >>>>>> --- a/arch/loongarch/include/asm/qspinlock.h
> >>>>>> +++ b/arch/loongarch/include/asm/qspinlock.h
> >>>>>> @@ -34,6 +34,11 @@ static inline bool virt_spin_lock(struct qspinlock *lock)
> >>>>>>            return true;
> >>>>>>     }
> >>>>>>
> >>>>>> +#ifdef CONFIG_SMP
> >>>>>> +#define vcpu_is_preempted      vcpu_is_preempted
> >>>>>> +bool vcpu_is_preempted(int cpu);
> >>>>> In V1 there is a build error because you reference mp_ops, so in V2
> >>>>> you needn't put it in CONFIG_SMP.
> >>>> The compile failure problem is that vcpu_is_preempted() is redefined in
> >>>> both arch/loongarch/kernel/paravirt.c and include/linux/sched.h
> >>> But other archs don't define vcpu_is_preempted() under CONFIG_SMP, and
> >> so what is advantage to implement this function if CONFIG_SMP is disabled?
> > 1. Keep consistency with other architectures.
> > 2. Keep it simple to reduce #ifdefs (and !SMP is just for build, not
> > very useful in practice).
> It seems that CONFIG_SMP can be removed in header file
> include/asm/qspinlock.h, since asm/spinlock.h and asm/qspinlock.h is
> only included when CONFIG_SMP is set, otherwise only linux/spinlock_up.h
> is included.
>
> >
> >>
> >>> you can consider to inline the whole vcpu_is_preempted() here.
> >> Defining the function vcpu_is_preempted() as inlined is not so easy for
> >> me, it beyond my ability now :(
> >>
> >> With static key method, the static key need be exported, all modules
> >> need apply the jump label, that is dangerous and I doubt whether it is
> >> deserved.
> > No, you have already done similar things in virt_spin_lock(), it is an
> > inline function and uses virt_spin_lock_key.
> virt_spin_lock is only called qspinlock in function
> queued_spin_lock_slowpath(). Function vcpu_is_preempted() is defined
> header file linux/sched.h, kernel module may use it.
Yes, if modules want to use it we need to EXPORT_SYMBOL. But don't
worry, static key infrastructure can handle this. Please see
cpu_feature_keys defined and used in
arch/powerpc/include/asm/cpu_has_feature.h, which is exported in
arch/powerpc/kernel/cputable.c.

Huacai

>
>
> >
> > Huacai
> >
> >>
> >> Regards
> >> Bibo Mao
> >>>
> >>>>
> >>>> The problem is that <asm/spinlock.h> is not included by sched.h, if
> >>>> CONFIG_SMP is disabled. Here is part of file include/linux/spinlock.h
> >>>> #ifdef CONFIG_SMP
> >>>> # include <asm/spinlock.h>
> >>>> #else
> >>>> # include <linux/spinlock_up.h>
> >>>> #endif
> >>>>
> >>>>> On the other hand, even if you really build a UP guest kernel, when
> >>>>> multiple guests run together, you probably need vcpu_is_preemtped.
> >>>> It is not relative with multiple VMs. When vcpu_is_preempted() is
> >>>> called, it is to detect whether dest CPU is preempted or not, the cpu
> >>>> from smp_processor_id() should not be preempted. So in generic
> >>>> vcpu_is_preempted() works on multiple vCPUs.
> >>> OK, I'm wrong here.
> >>>
> >>>
> >>> Huacai
> >>>
> >>>>
> >>>> Regards
> >>>> Bibo Mao
> >>>>>
> >>>>>
> >>>>> Huacai
> >>>>>
> >>>>>> +#endif
> >>>>>> +
> >>>>>>     #endif /* CONFIG_PARAVIRT */
> >>>>>>
> >>>>>>     #include <asm-generic/qspinlock.h>
> >>>>>> diff --git a/arch/loongarch/kernel/paravirt.c b/arch/loongarch/kernel/paravirt.c
> >>>>>> index b1b51f920b23..d4163679adc4 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
> >>>>>> +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);
> >>>>>> --
> >>>>>> 2.39.3
> >>>>>>
> >>>>
> >>>>
> >>
> >>
>
>