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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAAhV-H66M+GZ2kB8BKR82BUeQcNZ8ACeXLxwjh-bsVZcca1cqQ@mail.gmail.com>
Date: Mon, 24 Nov 2025 15:13:07 +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 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
you can consider to inline the whole vcpu_is_preempted() here.

>
> 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
> >>
>
>