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

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Message-ID: <CAAhV-H5iUqfBKS2ONv1VN8FrUUMwPc_ZNP6Ms5ufGKW2gD0twA@mail.gmail.com>
Date: Mon, 24 Nov 2025 17:33:38 +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 5:08 PM Bibo Mao <maobibo@...ngson.cn> wrote:
>
>
>
> On 2025/11/24 下午5:03, Huacai Chen wrote:
> > 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>steal_time
> >>>>>>>> ---
> >>>>>>>>      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.
> No, I do not want to do so. export static key and percpu steal_time> structure, just in order to implement one inline function.
In V1 you care about the performance of vcpu_is_preempted(), so inline
can satisfy your own requirement.

But this is your own choice, I don't insist on that. I only want to
remove CONFIG_SMP for vcpu_is_preempted().


Huacai
>
> >
> > 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
> >>>>>>>>
> >>>>>>
> >>>>>>
> >>>>
> >>>>
> >>
> >>
>
>