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Message-ID: <CAAhV-H5qZ3_KTvkZ-zQni6Lg-6W5y9oBXDb9+2VAeFV82BEzhA@mail.gmail.com>
Date: Tue, 18 Nov 2025 20:46:52 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Bibo Mao <maobibo@...ngson.cn>
Cc: Paolo Bonzini <pbonzini@...hat.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 1/3] LoongArch: KVM: Add preempt hint feature in
hypervisor side
Hi, Bibo,
On Tue, Nov 18, 2025 at 4:07 PM Bibo Mao <maobibo@...ngson.cn> wrote:
>
> Feature KVM_FEATURE_PREEMPT_HINT is added to show whether vCPU is
> preempted or not. It is to help guest OS scheduling or lock checking
> etc. Here add KVM_FEATURE_PREEMPT_HINT feature and use one byte as
> preempted flag in steal time structure.
>
> Signed-off-by: Bibo Mao <maobibo@...ngson.cn>
> ---
> arch/loongarch/include/asm/kvm_host.h | 2 +
> arch/loongarch/include/asm/kvm_para.h | 5 +-
> arch/loongarch/include/uapi/asm/kvm.h | 1 +
> arch/loongarch/include/uapi/asm/kvm_para.h | 1 +
> arch/loongarch/kvm/vcpu.c | 54 +++++++++++++++++++++-
> arch/loongarch/kvm/vm.c | 5 +-
> 6 files changed, 65 insertions(+), 3 deletions(-)
>
> diff --git a/arch/loongarch/include/asm/kvm_host.h b/arch/loongarch/include/asm/kvm_host.h
> index 0cecbd038bb3..04c6dd171877 100644
> --- a/arch/loongarch/include/asm/kvm_host.h
> +++ b/arch/loongarch/include/asm/kvm_host.h
> @@ -163,6 +163,7 @@ enum emulation_result {
> #define LOONGARCH_PV_FEAT_UPDATED BIT_ULL(63)
> #define LOONGARCH_PV_FEAT_MASK (BIT(KVM_FEATURE_IPI) | \
> BIT(KVM_FEATURE_STEAL_TIME) | \
> + BIT(KVM_FEATURE_PREEMPT_HINT) |\
> BIT(KVM_FEATURE_USER_HCALL) | \
> BIT(KVM_FEATURE_VIRT_EXTIOI))
>
> @@ -250,6 +251,7 @@ struct kvm_vcpu_arch {
> u64 guest_addr;
> u64 last_steal;
> struct gfn_to_hva_cache cache;
> + u8 preempted;
> } st;
> };
>
> diff --git a/arch/loongarch/include/asm/kvm_para.h b/arch/loongarch/include/asm/kvm_para.h
> index 3e4b397f423f..d8592a7f5922 100644
> --- a/arch/loongarch/include/asm/kvm_para.h
> +++ b/arch/loongarch/include/asm/kvm_para.h
> @@ -37,8 +37,11 @@ struct kvm_steal_time {
> __u64 steal;
> __u32 version;
> __u32 flags;
> - __u32 pad[12];
> + __u8 preempted;
> + __u8 u8_pad[3];
> + __u32 pad[11];
Maybe a single __u8 pad[47] is enough?
> };
> +#define KVM_VCPU_PREEMPTED (1 << 0)
>
> /*
> * Hypercall interface for KVM hypervisor
> diff --git a/arch/loongarch/include/uapi/asm/kvm.h b/arch/loongarch/include/uapi/asm/kvm.h
> index 57ba1a563bb1..bca7154aa651 100644
> --- a/arch/loongarch/include/uapi/asm/kvm.h
> +++ b/arch/loongarch/include/uapi/asm/kvm.h
> @@ -104,6 +104,7 @@ struct kvm_fpu {
> #define KVM_LOONGARCH_VM_FEAT_PV_IPI 6
> #define KVM_LOONGARCH_VM_FEAT_PV_STEALTIME 7
> #define KVM_LOONGARCH_VM_FEAT_PTW 8
> +#define KVM_LOONGARCH_VM_FEAT_PV_PREEMPT_HINT 10
>From the name it is a "hint", from include/linux/kvm_para.h we know
features and hints are different. If preempt is really a feature,
rename it?
>
> /* Device Control API on vcpu fd */
> #define KVM_LOONGARCH_VCPU_CPUCFG 0
> diff --git a/arch/loongarch/include/uapi/asm/kvm_para.h b/arch/loongarch/include/uapi/asm/kvm_para.h
> index 76d802ef01ce..fe4107869ce6 100644
> --- a/arch/loongarch/include/uapi/asm/kvm_para.h
> +++ b/arch/loongarch/include/uapi/asm/kvm_para.h
> @@ -15,6 +15,7 @@
> #define CPUCFG_KVM_FEATURE (CPUCFG_KVM_BASE + 4)
> #define KVM_FEATURE_IPI 1
> #define KVM_FEATURE_STEAL_TIME 2
> +#define KVM_FEATURE_PREEMPT_HINT 3
> /* BIT 24 - 31 are features configurable by user space vmm */
> #define KVM_FEATURE_VIRT_EXTIOI 24
> #define KVM_FEATURE_USER_HCALL 25
> diff --git a/arch/loongarch/kvm/vcpu.c b/arch/loongarch/kvm/vcpu.c
> index 1245a6b35896..33a94b191b5d 100644
> --- a/arch/loongarch/kvm/vcpu.c
> +++ b/arch/loongarch/kvm/vcpu.c
> @@ -180,6 +180,11 @@ static void kvm_update_stolen_time(struct kvm_vcpu *vcpu)
> }
>
> st = (struct kvm_steal_time __user *)ghc->hva;
> + if (kvm_guest_has_pv_feature(vcpu, KVM_FEATURE_PREEMPT_HINT)) {
> + unsafe_put_user(0, &st->preempted, out);
> + vcpu->arch.st.preempted = 0;
> + }
> +
> unsafe_get_user(version, &st->version, out);
> if (version & 1)
> version += 1; /* first time write, random junk */
> @@ -1757,11 +1762,58 @@ static int _kvm_vcpu_put(struct kvm_vcpu *vcpu, int cpu)
> return 0;
> }
>
> +static void _kvm_set_vcpu_preempted(struct kvm_vcpu *vcpu)
Just using kvm_set_vcpu_preempted() is enough, no "_".
> +{
> + struct gfn_to_hva_cache *ghc;
> + struct kvm_steal_time __user *st;
> + struct kvm_memslots *slots;
> + static const u8 preempted = KVM_VCPU_PREEMPTED;
I'm not sure whether "static" is right, it's not reentrant.
Huacai
> + gpa_t gpa;
> +
> + gpa = vcpu->arch.st.guest_addr;
> + if (!(gpa & KVM_STEAL_PHYS_VALID))
> + return;
> +
> + /* vCPU may be preempted for many times */
> + if (vcpu->arch.st.preempted)
> + return;
> +
> + /* This happens on process exit */
> + if (unlikely(current->mm != vcpu->kvm->mm))
> + return;
> +
> + gpa &= KVM_STEAL_PHYS_MASK;
> + ghc = &vcpu->arch.st.cache;
> + slots = kvm_memslots(vcpu->kvm);
> + if (slots->generation != ghc->generation || gpa != ghc->gpa) {
> + if (kvm_gfn_to_hva_cache_init(vcpu->kvm, ghc, gpa, sizeof(*st))) {
> + ghc->gpa = INVALID_GPA;
> + return;
> + }
> + }
> +
> + st = (struct kvm_steal_time __user *)ghc->hva;
> + unsafe_put_user(preempted, &st->preempted, out);
> + vcpu->arch.st.preempted = KVM_VCPU_PREEMPTED;
> +out:
> + mark_page_dirty_in_slot(vcpu->kvm, ghc->memslot, gpa_to_gfn(ghc->gpa));
> +}
> +
> void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> {
> - int cpu;
> + int cpu, idx;
> unsigned long flags;
>
> + if (vcpu->preempted && kvm_guest_has_pv_feature(vcpu, KVM_FEATURE_PREEMPT_HINT)) {
> + /*
> + * Take the srcu lock as memslots will be accessed to check the gfn
> + * cache generation against the memslots generation.
> + */
> + idx = srcu_read_lock(&vcpu->kvm->srcu);
> + _kvm_set_vcpu_preempted(vcpu);
> + srcu_read_unlock(&vcpu->kvm->srcu, idx);
> + }
> +
> local_irq_save(flags);
> cpu = smp_processor_id();
> vcpu->arch.last_sched_cpu = cpu;
> diff --git a/arch/loongarch/kvm/vm.c b/arch/loongarch/kvm/vm.c
> index a49b1c1a3dd1..b8879110a0a1 100644
> --- a/arch/loongarch/kvm/vm.c
> +++ b/arch/loongarch/kvm/vm.c
> @@ -45,8 +45,10 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>
> /* Enable all PV features by default */
> kvm->arch.pv_features = BIT(KVM_FEATURE_IPI);
> - if (kvm_pvtime_supported())
> + if (kvm_pvtime_supported()) {
> kvm->arch.pv_features |= BIT(KVM_FEATURE_STEAL_TIME);
> + kvm->arch.pv_features |= BIT(KVM_FEATURE_PREEMPT_HINT);
> + }
>
> /*
> * cpu_vabits means user address space only (a half of total).
> @@ -143,6 +145,7 @@ static int kvm_vm_feature_has_attr(struct kvm *kvm, struct kvm_device_attr *attr
> case KVM_LOONGARCH_VM_FEAT_PV_IPI:
> return 0;
> case KVM_LOONGARCH_VM_FEAT_PV_STEALTIME:
> + case KVM_LOONGARCH_VM_FEAT_PV_PREEMPT_HINT:
> if (kvm_pvtime_supported())
> return 0;
> return -ENXIO;
> --
> 2.39.3
>
>
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