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Message-ID: <f8ad1ecb-0272-f027-807c-98d2511c2871@loongson.cn>
Date: Wed, 19 Nov 2025 10:55:33 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
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
On 2025/11/19 上午10:45, Huacai Chen wrote:
> On Wed, Nov 19, 2025 at 9:23 AM Bibo Mao <maobibo@...ngson.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@...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?
>> 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
>>>>
>>>>
>>
>>
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