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@loongson.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];
};
+#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
/* 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)
+{
+ struct gfn_to_hva_cache *ghc;
+ struct kvm_steal_time __user *st;
+ struct kvm_memslots *slots;
+ static const u8 preempted = KVM_VCPU_PREEMPTED;
+ 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;
--
2.39.3Hi, Bibo,
On Tue, Nov 18, 2025 at 4:07 PM Bibo Mao <maobibo@loongson.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@loongson.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
>
>
On 2025/11/18 下午8:46, Huacai Chen wrote:
> Hi, Bibo,
>
> On Tue, Nov 18, 2025 at 4:07 PM Bibo Mao <maobibo@loongson.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@loongson.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?
yes, pad[47] seems better unless there is definitely __u32 type
requirement in future.
Will do in next version.
>
>> };
>> +#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?
It is a feature. yes, in generic hint is suggestion for VM and VM can
selectively do or not.
Will rename it with KVM_LOONGARCH_VM_FEAT_PV_PREEMPT.
>
>>
>> /* 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.
I think static is better here, it saves one cycle with assignment here.
Regards
Bibo Mao
>
>
> 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
>>
>>
On Wed, Nov 19, 2025 at 9:23 AM Bibo Mao <maobibo@loongson.cn> wrote:
>
>
>
> On 2025/11/18 下午8:46, Huacai Chen wrote:
> > Hi, Bibo,
> >
> > On Tue, Nov 18, 2025 at 4:07 PM Bibo Mao <maobibo@loongson.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@loongson.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?
> yes, pad[47] seems better unless there is definitely __u32 type
> requirement in future.
>
> Will do in next version.
> >
> >> };
> >> +#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?
> It is a feature. yes, in generic hint is suggestion for VM and VM can
> selectively do or not.
>
> Will rename it with KVM_LOONGARCH_VM_FEAT_PV_PREEMPT.
> >
> >>
> >> /* 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.
> I think static is better here, it saves one cycle with assignment here.
I know, but I want to know whether the logic is correct.
vcpu->arch.st.preempted is per-cpu, but the local variable "preempted"
can be used across multiple VCPU? I'm not sure.
Huacai
>
> Regards
> Bibo Mao
> >
> >
> > 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
> >>
> >>
>
>
On 2025/11/19 上午10:45, Huacai Chen wrote:
> On Wed, Nov 19, 2025 at 9:23 AM Bibo Mao <maobibo@loongson.cn> wrote:
>>
>>
>>
>> On 2025/11/18 下午8:46, Huacai Chen wrote:
>>> Hi, Bibo,
>>>
>>> On Tue, Nov 18, 2025 at 4:07 PM Bibo Mao <maobibo@loongson.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@loongson.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?
>> yes, pad[47] seems better unless there is definitely __u32 type
>> requirement in future.
>>
>> Will do in next version.
>>>
>>>> };
>>>> +#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?
>> It is a feature. yes, in generic hint is suggestion for VM and VM can
>> selectively do or not.
>>
>> Will rename it with KVM_LOONGARCH_VM_FEAT_PV_PREEMPT.
>>>
>>>>
>>>> /* 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.
>> I think static is better here, it saves one cycle with assignment here.
> I know, but I want to know whether the logic is correct.
> vcpu->arch.st.preempted is per-cpu, but the local variable "preempted"
> can be used across multiple VCPU? I'm not sure.
It is read-only, of course can be used by multiple vCPUs. or remove it
directly?
@@ -1767,7 +1767,6 @@ static void _kvm_set_vcpu_preempted(struct
kvm_vcpu *vcpu)
struct gfn_to_hva_cache *ghc;
struct kvm_steal_time __user *st;
struct kvm_memslots *slots;
- static const u8 preempted = KVM_VCPU_PREEMPTED;
gpa_t gpa;
gpa = vcpu->arch.st.guest_addr;
@@ -1793,7 +1792,7 @@ static void _kvm_set_vcpu_preempted(struct
kvm_vcpu *vcpu)
}
st = (struct kvm_steal_time __user *)ghc->hva;
- unsafe_put_user(preempted, &st->preempted, out);
+ unsafe_put_user(KVM_VCPU_PREEMPTED, &st->preempted, out);
vcpu->arch.st.preempted = KVM_VCPU_PREEMPTED;
>
> Huacai
>
>>
>> Regards
>> Bibo Mao
>>>
>>>
>>> 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
>>>>
>>>>
>>
>>
On Wed, Nov 19, 2025 at 10:58 AM Bibo Mao <maobibo@loongson.cn> wrote:
>
>
>
> On 2025/11/19 上午10:45, Huacai Chen wrote:
> > On Wed, Nov 19, 2025 at 9:23 AM Bibo Mao <maobibo@loongson.cn> wrote:
> >>
> >>
> >>
> >> On 2025/11/18 下午8:46, Huacai Chen wrote:
> >>> Hi, Bibo,
> >>>
> >>> On Tue, Nov 18, 2025 at 4:07 PM Bibo Mao <maobibo@loongson.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@loongson.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?
> >> yes, pad[47] seems better unless there is definitely __u32 type
> >> requirement in future.
> >>
> >> Will do in next version.
> >>>
> >>>> };
> >>>> +#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?
> >> It is a feature. yes, in generic hint is suggestion for VM and VM can
> >> selectively do or not.
> >>
> >> Will rename it with KVM_LOONGARCH_VM_FEAT_PV_PREEMPT.
> >>>
> >>>>
> >>>> /* 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.
> >> I think static is better here, it saves one cycle with assignment here.
> > I know, but I want to know whether the logic is correct.
> > vcpu->arch.st.preempted is per-cpu, but the local variable "preempted"
> > can be used across multiple VCPU? I'm not sure.
> It is read-only, of course can be used by multiple vCPUs. or remove it
> directly?
Good, remove it directly.
Huacai
>
> @@ -1767,7 +1767,6 @@ static void _kvm_set_vcpu_preempted(struct
> kvm_vcpu *vcpu)
> struct gfn_to_hva_cache *ghc;
> struct kvm_steal_time __user *st;
> struct kvm_memslots *slots;
> - static const u8 preempted = KVM_VCPU_PREEMPTED;
> gpa_t gpa;
>
> gpa = vcpu->arch.st.guest_addr;
> @@ -1793,7 +1792,7 @@ static void _kvm_set_vcpu_preempted(struct
> kvm_vcpu *vcpu)
> }
>
> st = (struct kvm_steal_time __user *)ghc->hva;
> - unsafe_put_user(preempted, &st->preempted, out);
> + unsafe_put_user(KVM_VCPU_PREEMPTED, &st->preempted, out);
> vcpu->arch.st.preempted = KVM_VCPU_PREEMPTED;
>
> >
> > Huacai
> >
> >>
> >> Regards
> >> Bibo Mao
> >>>
> >>>
> >>> 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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