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Message-ID: <CAAhV-H7ij5pcr6i6q-eh-2nUzs-L2Wq79PaxMnE_t5a4w7=75A@mail.gmail.com>
Date: Wed, 19 Nov 2025 15:41:54 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Bibo Mao <maobibo@...ngson.cn>
Cc: Paolo Bonzini <pbonzini@...hat.com>, WANG Xuerui <kernel@...0n.name>,
Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
Boqun Feng <boqun.feng@...il.com>, Waiman Long <longman@...hat.com>,
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 2/3] LoongArch: Add paravirt support with vcpu_is_preempted()
On Wed, Nov 19, 2025 at 2:12 PM Bibo Mao <maobibo@...ngson.cn> wrote:
>
>
>
> On 2025/11/19 上午9:59, Bibo Mao wrote:
> >
> >
> > On 2025/11/18 下午8:48, Huacai Chen wrote:
> >> Hi, Bibo,
> >>
> >> On Tue, Nov 18, 2025 at 4:07 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>
> >>> ---
> >>> arch/loongarch/include/asm/smp.h | 1 +
> >>> arch/loongarch/include/asm/spinlock.h | 5 +++++
> >>> arch/loongarch/kernel/paravirt.c | 16 ++++++++++++++++
> >>> arch/loongarch/kernel/smp.c | 6 ++++++
> >>> 4 files changed, 28 insertions(+)
> >>>
> >>> diff --git a/arch/loongarch/include/asm/smp.h
> >>> b/arch/loongarch/include/asm/smp.h
> >>> index 3a47f52959a8..5b37f7bf2060 100644
> >>> --- a/arch/loongarch/include/asm/smp.h
> >>> +++ b/arch/loongarch/include/asm/smp.h
> >>> @@ -18,6 +18,7 @@ struct smp_ops {
> >>> void (*init_ipi)(void);
> >>> void (*send_ipi_single)(int cpu, unsigned int action);
> >>> void (*send_ipi_mask)(const struct cpumask *mask, unsigned
> >>> int action);
> >>> + bool (*vcpu_is_preempted)(int cpu);
> >>> };
> >>> extern struct smp_ops mp_ops;
> >>>
> >>> diff --git a/arch/loongarch/include/asm/spinlock.h
> >>> b/arch/loongarch/include/asm/spinlock.h
> >>> index 7cb3476999be..c001cef893aa 100644
> >>> --- a/arch/loongarch/include/asm/spinlock.h
> >>> +++ b/arch/loongarch/include/asm/spinlock.h
> >>> @@ -5,6 +5,11 @@
> >>> #ifndef _ASM_SPINLOCK_H
> >>> #define _ASM_SPINLOCK_H
> >>>
> >>> +#ifdef CONFIG_PARAVIRT
> >>> +#define vcpu_is_preempted vcpu_is_preempted
> >>> +bool vcpu_is_preempted(int cpu);
> >>> +#endif
> >> Maybe paravirt.h is a better place?
> >
> > It is actually a little strange to add macro CONFIG_PARAVIRT in file
> > asm/spinlock.h
> >
> > vcpu_is_preempted is originally defined in header file
> > include/linux/sched.h like this
> > #ifndef vcpu_is_preempted
> > static inline bool vcpu_is_preempted(int cpu)
> > {
> > return false;
> > }
> > #endif
> >
> > that requires that header file is included before sched.h, file
> > asm/spinlock.h can meet this requirement, however header file paravirt.h
> > maybe it is not included before sched.h in generic.
> >
> > Here vcpu_is_preempted definition is added before the following including.
> > #include <asm/processor.h>
> > #include <asm/qspinlock.h>
> > #include <asm/qrwlock.h>
> > Maybe it is better to be added after the above header files including
> > sentences, but need further investigation.
> >>
> >>> +
> >>> #include <asm/processor.h>
> >>> #include <asm/qspinlock.h>
> >>> #include <asm/qrwlock.h>
> >>> diff --git a/arch/loongarch/kernel/paravirt.c
> >>> b/arch/loongarch/kernel/paravirt.c
> >>> index b1b51f920b23..b99404b6b13f 100644
> >>> --- a/arch/loongarch/kernel/paravirt.c
> >>> +++ b/arch/loongarch/kernel/paravirt.c
> >>> @@ -52,6 +52,13 @@ static u64 paravt_steal_clock(int cpu)
> >>> #ifdef CONFIG_SMP
> >>> static struct smp_ops native_ops;
> >>>
> >>> +static bool pv_vcpu_is_preempted(int cpu)
> >>> +{
> >>> + struct kvm_steal_time *src = &per_cpu(steal_time, cpu);
> >>> +
> >>> + return !!(src->preempted & KVM_VCPU_PREEMPTED);
> >>> +}
> >>> +
> >>> static void pv_send_ipi_single(int cpu, unsigned int action)
> >>> {
> >>> int min, old;
> >>> @@ -308,6 +315,9 @@ 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_HINT))
> >>> + mp_ops.vcpu_is_preempted = pv_vcpu_is_preempted;
> >>> #endif
> >>>
> >>> static_call_update(pv_steal_clock, paravt_steal_clock);
> >>> @@ -332,3 +342,9 @@ int __init pv_spinlock_init(void)
> >>>
> >>> return 0;
> >>> }
> >>> +
> >>> +bool notrace vcpu_is_preempted(int cpu)
> >>> +{
> >>> + return mp_ops.vcpu_is_preempted(cpu);
> >>> +}
> >>
> >> We can simplify the whole patch like this, then we don't need to touch
> >> smp.c, and we can merge Patch-2/3.
> >>
> >> +bool notrace vcpu_is_preempted(int cpu)
> >> +{
> >> + if (!kvm_para_has_feature(KVM_FEATURE_PREEMPT_HINT))
> >> + return false;
> >> + else {
> >> + struct kvm_steal_time *src = &per_cpu(steal_time, cpu);
> >> + return !!(src->preempted & KVM_VCPU_PREEMPTED);
> >> + }
> >> +}
> > 1. there is assembly output about relative vcpu_is_preempted
> > <loongson_vcpu_is_preempted>:
> > move $r4,$r0
> > jirl $r0,$r1,0
> >
> > <pv_vcpu_is_preempted>:
> > pcalau12i $r13,8759(0x2237)
> > slli.d $r4,$r4,0x3
> > addi.d $r13,$r13,-1000(0xc18)
> > ldx.d $r13,$r13,$r4
> > pcalau12i $r12,5462(0x1556)
> > addi.d $r12,$r12,384(0x180)
> > add.d $r12,$r13,$r12
> > ld.bu $r4,$r12,16(0x10)
> > andi $r4,$r4,0x1
> > jirl $r0,$r1,0
> >
> > <vcpu_is_preempted>:
> > pcalau12i $r12,8775(0x2247)
> > ld.d $r12,$r12,-472(0xe28)
> > jirl $r0,$r12,0
> > andi $r0,$r0,0x0
> >
> > <vcpu_is_preempted_new>:
> > pcalau12i $r12,8151(0x1fd7)
> > ld.d $r12,$r12,-1008(0xc10)
> > bstrpick.d $r12,$r12,0x1a,0x1a
> > beqz $r12,188(0xbc) # 900000000024ec60
> > pcalau12i $r12,11802(0x2e1a)
> > addi.d $r12,$r12,-1400(0xa88)
> > ldptr.w $r14,$r12,36(0x24)
> > beqz $r14,108(0x6c) # 900000000024ec20
> > addi.w $r13,$r0,1(0x1)
> > bne $r14,$r13,164(0xa4) # 900000000024ec60
> > ldptr.w $r13,$r12,40(0x28)
> > bnez $r13,24(0x18) # 900000000024ebdc
> > lu12i.w $r14,262144(0x40000)
> > ori $r14,$r14,0x4
> > cpucfg $r14,$r14
> > slli.w $r13,$r14,0x0
> > st.w $r14,$r12,40(0x28)
> > bstrpick.d $r13,$r13,0x3,0x3
> > beqz $r13,128(0x80) # 900000000024ec60
> > pcalau12i $r13,8759(0x2237)
> > slli.d $r4,$r4,0x3
> > addi.d $r13,$r13,-1000(0xc18)
> > ldx.d $r13,$r13,$r4
> > pcalau12i $r12,5462(0x1556)
> > addi.d $r12,$r12,384(0x180)
> > add.d $r12,$r13,$r12
> > ld.bu $r4,$r12,16(0x10)
> > andi $r4,$r4,0x1
> > jirl $r0,$r1,0
> > andi $r0,$r0,0x0
> > andi $r0,$r0,0x0
> > andi $r0,$r0,0x0
> > andi $r0,$r0,0x0
> > andi $r0,$r0,0x0
> > lu12i.w $r13,262144(0x40000)
> > cpucfg $r13,$r13
> > lu12i.w $r15,1237(0x4d5)
> > ori $r15,$r15,0x64b
> > slli.w $r13,$r13,0x0
> > bne $r13,$r15,-124(0x3ff84) # 900000000024ebb8
> > addi.w $r13,$r0,1(0x1)
> > st.w $r13,$r12,36(0x24)
> > b -128(0xfffff80) # 900000000024ebc0
> > andi $r0,$r0,0x0
> > andi $r0,$r0,0x0
> > andi $r0,$r0,0x0
> > andi $r0,$r0,0x0
> > andi $r0,$r0,0x0
> > andi $r0,$r0,0x0
> > andi $r0,$r0,0x0
> > move $r4,$r0
> > jirl $r0,$r1,0
> >
> > With vcpu_is_preempted(), there is one memory load and one jirl jump,
> > with vcpu_is_preempted_new(), there is two memory load and two beq
> > compare instructions.
> >
> > 2. In some scenery such nr_cpus == 1, loongson_vcpu_is_preempted() is
> > better than pv_vcpu_is_preempted() even if the preempt feature is enabled.
> how about use static key and keep file smp.c untouched?
OK, it's better.
Huacai
> 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);
> }
>
> it reduces one memory load, here is assembly output:
> <vcpu_is_preempted>:
> andi $r0,$r0,0x0
> move $r4,$r0
> jirl $r0,$r1,0
> andi $r0,$r0,0x0
> pcalau12i $r13,8759(0x2237)
> slli.d $r4,$r4,0x3
> addi.d $r13,$r13,-1000(0xc18)
> ldx.d $r13,$r13,$r4
> pcalau12i $r12,5462(0x1556)
> addi.d $r12,$r12,384(0x180)
> add.d $r12,$r13,$r12
> ld.bu $r4,$r12,16(0x10)
> andi $r4,$r4,0x1
> jirl $r0,$r1,0
>
> Regards
> Bibo Mao
>
> >
> > Regards
> > Bibo Mao
> >> Huacai
> >>
> >>> +EXPORT_SYMBOL(vcpu_is_preempted);
> >>> diff --git a/arch/loongarch/kernel/smp.c b/arch/loongarch/kernel/smp.c
> >>> index 46036d98da75..f04192fedf8d 100644
> >>> --- a/arch/loongarch/kernel/smp.c
> >>> +++ b/arch/loongarch/kernel/smp.c
> >>> @@ -307,10 +307,16 @@ static void loongson_init_ipi(void)
> >>> panic("IPI IRQ request failed\n");
> >>> }
> >>>
> >>> +static bool loongson_vcpu_is_preempted(int cpu)
> >>> +{
> >>> + return false;
> >>> +}
> >>> +
> >>> struct smp_ops mp_ops = {
> >>> .init_ipi = loongson_init_ipi,
> >>> .send_ipi_single = loongson_send_ipi_single,
> >>> .send_ipi_mask = loongson_send_ipi_mask,
> >>> + .vcpu_is_preempted = loongson_vcpu_is_preempted,
> >>> };
> >>>
> >>> static void __init fdt_smp_setup(void)
> >>> --
> >>> 2.39.3
> >>>
> >>>
> >
>
>
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