linux-kernel - Re: [PATCH v4 2/2] LoongArch: Add paravirt support with vcpu_is

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Message-ID: <CAAhV-H6mD1OxeVDaAXw_y6+1mHBwgGzS4OpsX4aezxV0SP8KLw@mail.gmail.com>
Date: Fri, 30 Jan 2026 17:56:16 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Bibo Mao <maobibo@...ngson.cn>
Cc: Juergen Gross <jgross@...e.com>, Tianrui Zhao <zhaotianrui@...ngson.cn>, 
	WANG Xuerui <kernel@...0n.name>, kvm@...r.kernel.org, loongarch@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] LoongArch: Add paravirt support with
 vcpu_is_preempted() in guest side

On Fri, Jan 30, 2026 at 9:24 AM Bibo Mao <maobibo@...ngson.cn> wrote:
>
>
>
> On 2026/1/29 下午8:55, Huacai Chen wrote:
> > Hi, Bibo,
> >
> > On Fri, Dec 19, 2025 at 2:30 PM 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>
> >> Acked-by: Juergen Gross <jgross@...e.com>
> >> ---
> >>   arch/loongarch/include/asm/qspinlock.h |  3 +++
> >>   arch/loongarch/kernel/paravirt.c       | 21 ++++++++++++++++++++-
> >>   2 files changed, 23 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..a81a3e871dd1 100644
> >> --- a/arch/loongarch/kernel/paravirt.c
> >> +++ b/arch/loongarch/kernel/paravirt.c
> >> @@ -12,6 +12,7 @@ static int has_steal_clock;
> >>   struct static_key paravirt_steal_enabled;
> >>   struct static_key paravirt_steal_rq_enabled;
> >>   static DEFINE_PER_CPU(struct kvm_steal_time, steal_time) __aligned(64);
> >> +static DEFINE_STATIC_KEY_FALSE(virt_preempt_key);
> >>   DEFINE_STATIC_KEY_FALSE(virt_spin_lock_key);
> >>
> >>   static u64 native_steal_clock(int cpu)
> >> @@ -267,6 +268,18 @@ static int pv_time_cpu_down_prepare(unsigned int cpu)
> >>
> >>          return 0;
> >>   }
> >> +
> >> +bool notrace vcpu_is_preempted(int cpu)
> > Is "notrace" really needed? Only S390 do this.
>
> The prefix "notrace" is copied from S390, it is inline function on x86.
>
> Here is git log information with arch/s390/kernel/smp.c
> commit 8ebf6da9db1b2a20bb86cc1bee2552e894d03308
> Author: Philipp Rudo <prudo@...ux.ibm.com>
> Date:   Mon Apr 6 20:47:48 2020
>
>      s390/ftrace: fix potential crashes when switching tracers
>
>      Switching tracers include instruction patching. To prevent that a
>      instruction is patched while it's read the instruction patching is done
>      in stop_machine 'context'. This also means that any function called
>      during stop_machine must not be traced. Thus add 'notrace' to all
>      functions called within stop_machine.
>
>      Fixes: 1ec2772e0c3c ("s390/diag: add a statistic for diagnose calls")
>      Fixes: 38f2c691a4b3 ("s390: improve wait logic of stop_machine")
>      Fixes: 4ecf0a43e729 ("processor: get rid of cpu_relax_yield")
>      Signed-off-by: Philipp Rudo <prudo@...ux.ibm.com>
>      Signed-off-by: Vasily Gorbik <gor@...ux.ibm.com>
>
> However I am not familiar with tracer and have no idea about this, that
> is both OK to me. You are Linux kernel expert, what is your opinion
> about notrace prefix?
It seems only S390 calls vcpu_is_preempted() in stop_machine(), and on
X86 both __kvm_vcpu_is_preempted() and __native_vcpu_is_preempted()
have no "notrace", so I applied this patch and drop "notrace".

Huacai

>
> Regards
> Bibo Mao
> >
> > Huacai
> >
> >> +{
> >> +       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 +334,10 @@ int __init pv_time_init(void)
> >>                  static_key_slow_inc(&paravirt_steal_rq_enabled);
> >>   #endif
> >>
> >> -       pr_info("Using paravirt steal-time\n");
> >> +       if (static_key_enabled(&virt_preempt_key))
> >> +               pr_info("Using paravirt steal-time with preempt enabled\n");
> >> +       else
> >> +               pr_info("Using paravirt steal-time with preempt disabled\n");
> >>
> >>          return 0;
> >>   }
> >> --
> >> 2.39.3
> >>
>
>