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Message-ID: <67db8a93-eb62-7278-ae3f-a46a62365b20@loongson.cn>
Date: Wed, 28 Jan 2026 14:33:13 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: WANG Xuerui <kernel@...0n.name>, Tianrui Zhao <zhaotianrui@...ngson.cn>,
loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 3/3] LoongArch: KVM: Add FPU delay load support
On 2026/1/28 下午12:05, Huacai Chen wrote:
> Hi, Bibo,
>
> On Tue, Jan 27, 2026 at 8:51 PM Bibo Mao <maobibo@...ngson.cn> wrote:
>>
>> FPU is lazy enabled with KVM hypervisor. After FPU is enabled and
>> loaded, vCPU can be preempted and FPU will be lost again. Here FPU
>> is delay load until guest enter entry.
>>
>> Signed-off-by: Bibo Mao <maobibo@...ngson.cn>
>> ---
>> arch/loongarch/include/asm/kvm_host.h | 2 ++
>> arch/loongarch/kvm/exit.c | 15 ++++++++----
>> arch/loongarch/kvm/vcpu.c | 33 +++++++++++++++++----------
>> 3 files changed, 33 insertions(+), 17 deletions(-)
>>
>> diff --git a/arch/loongarch/include/asm/kvm_host.h b/arch/loongarch/include/asm/kvm_host.h
>> index e4fe5b8e8149..902ff7bc0e35 100644
>> --- a/arch/loongarch/include/asm/kvm_host.h
>> +++ b/arch/loongarch/include/asm/kvm_host.h
>> @@ -37,6 +37,7 @@
>> #define KVM_REQ_TLB_FLUSH_GPA KVM_ARCH_REQ(0)
>> #define KVM_REQ_STEAL_UPDATE KVM_ARCH_REQ(1)
>> #define KVM_REQ_PMU KVM_ARCH_REQ(2)
>> +#define KVM_REQ_FPU_LOAD KVM_ARCH_REQ(3)
>>
>> #define KVM_GUESTDBG_SW_BP_MASK \
>> (KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_USE_SW_BP)
>> @@ -234,6 +235,7 @@ struct kvm_vcpu_arch {
>> u64 vpid;
>> gpa_t flush_gpa;
>>
>> + int fpu_load_type;
> I think the logic of V1 is better, it doesn't increase the size of
> kvm_vcpu_arch, and the constant checking is a little faster than
> variable checking.
The main reason is that FPU_LOAD request is not so frequent, there is
atomic instruction in kvm_check_request() and the unconditional
kvm_check_request() may be unnecessary, also there will LBT LOAD check
in later version.
So I think one unconditional atomic test_and_clear may be better than
three/four atomic test_and_clear.
kvm_check_request(KVM_REQ_FPU_LOAD,vcpu)
kvm_check_request(KVM_REQ_FPU_LSX, vcpu)
kvm_check_request(KVM_REQ_FPU_LASX, vcpu)
Actually different people have different view about this :)
Regards
Bibo Mao
>
> Huacai
>
>> /* Frequency of stable timer in Hz */
>> u64 timer_mhz;
>> ktime_t expire;
>> diff --git a/arch/loongarch/kvm/exit.c b/arch/loongarch/kvm/exit.c
>> index 74b427287e96..b6f08df8fedb 100644
>> --- a/arch/loongarch/kvm/exit.c
>> +++ b/arch/loongarch/kvm/exit.c
>> @@ -754,7 +754,8 @@ static int kvm_handle_fpu_disabled(struct kvm_vcpu *vcpu, int ecode)
>> return RESUME_HOST;
>> }
>>
>> - kvm_own_fpu(vcpu);
>> + vcpu->arch.fpu_load_type = KVM_LARCH_FPU;
>> + kvm_make_request(KVM_REQ_FPU_LOAD, vcpu);
>>
>> return RESUME_GUEST;
>> }
>> @@ -794,8 +795,10 @@ static int kvm_handle_lsx_disabled(struct kvm_vcpu *vcpu, int ecode)
>> {
>> if (!kvm_guest_has_lsx(&vcpu->arch))
>> kvm_queue_exception(vcpu, EXCCODE_INE, 0);
>> - else
>> - kvm_own_lsx(vcpu);
>> + else {
>> + vcpu->arch.fpu_load_type = KVM_LARCH_LSX;
>> + kvm_make_request(KVM_REQ_FPU_LOAD, vcpu);
>> + }
>>
>> return RESUME_GUEST;
>> }
>> @@ -812,8 +815,10 @@ static int kvm_handle_lasx_disabled(struct kvm_vcpu *vcpu, int ecode)
>> {
>> if (!kvm_guest_has_lasx(&vcpu->arch))
>> kvm_queue_exception(vcpu, EXCCODE_INE, 0);
>> - else
>> - kvm_own_lasx(vcpu);
>> + else {
>> + vcpu->arch.fpu_load_type = KVM_LARCH_LASX;
>> + kvm_make_request(KVM_REQ_FPU_LOAD, vcpu);
>> + }
>>
>> return RESUME_GUEST;
>> }
>> diff --git a/arch/loongarch/kvm/vcpu.c b/arch/loongarch/kvm/vcpu.c
>> index d91a1160a309..3e749e9738b2 100644
>> --- a/arch/loongarch/kvm/vcpu.c
>> +++ b/arch/loongarch/kvm/vcpu.c
>> @@ -232,6 +232,27 @@ static void kvm_late_check_requests(struct kvm_vcpu *vcpu)
>> kvm_flush_tlb_gpa(vcpu, vcpu->arch.flush_gpa);
>> vcpu->arch.flush_gpa = INVALID_GPA;
>> }
>> +
>> + if (kvm_check_request(KVM_REQ_FPU_LOAD, vcpu)) {
>> + switch (vcpu->arch.fpu_load_type) {
>> + case KVM_LARCH_FPU:
>> + kvm_own_fpu(vcpu);
>> + break;
>> +
>> + case KVM_LARCH_LSX:
>> + kvm_own_lsx(vcpu);
>> + break;
>> +
>> + case KVM_LARCH_LASX:
>> + kvm_own_lasx(vcpu);
>> + break;
>> +
>> + default:
>> + break;
>> + }
>> +
>> + vcpu->arch.fpu_load_type = 0;
>> + }
>> }
>>
>> /*
>> @@ -1338,8 +1359,6 @@ static inline void kvm_check_fcsr_alive(struct kvm_vcpu *vcpu) { }
>> /* Enable FPU and restore context */
>> void kvm_own_fpu(struct kvm_vcpu *vcpu)
>> {
>> - preempt_disable();
>> -
>> /*
>> * Enable FPU for guest
>> * Set FR and FRE according to guest context
>> @@ -1350,16 +1369,12 @@ void kvm_own_fpu(struct kvm_vcpu *vcpu)
>> kvm_restore_fpu(&vcpu->arch.fpu);
>> vcpu->arch.aux_inuse |= KVM_LARCH_FPU;
>> trace_kvm_aux(vcpu, KVM_TRACE_AUX_RESTORE, KVM_TRACE_AUX_FPU);
>> -
>> - preempt_enable();
>> }
>>
>> #ifdef CONFIG_CPU_HAS_LSX
>> /* Enable LSX and restore context */
>> int kvm_own_lsx(struct kvm_vcpu *vcpu)
>> {
>> - preempt_disable();
>> -
>> /* Enable LSX for guest */
>> kvm_check_fcsr(vcpu, vcpu->arch.fpu.fcsr);
>> set_csr_euen(CSR_EUEN_LSXEN | CSR_EUEN_FPEN);
>> @@ -1381,8 +1396,6 @@ int kvm_own_lsx(struct kvm_vcpu *vcpu)
>>
>> trace_kvm_aux(vcpu, KVM_TRACE_AUX_RESTORE, KVM_TRACE_AUX_LSX);
>> vcpu->arch.aux_inuse |= KVM_LARCH_LSX | KVM_LARCH_FPU;
>> - preempt_enable();
>> -
>> return 0;
>> }
>> #endif
>> @@ -1391,8 +1404,6 @@ int kvm_own_lsx(struct kvm_vcpu *vcpu)
>> /* Enable LASX and restore context */
>> int kvm_own_lasx(struct kvm_vcpu *vcpu)
>> {
>> - preempt_disable();
>> -
>> kvm_check_fcsr(vcpu, vcpu->arch.fpu.fcsr);
>> set_csr_euen(CSR_EUEN_FPEN | CSR_EUEN_LSXEN | CSR_EUEN_LASXEN);
>> switch (vcpu->arch.aux_inuse & (KVM_LARCH_FPU | KVM_LARCH_LSX)) {
>> @@ -1414,8 +1425,6 @@ int kvm_own_lasx(struct kvm_vcpu *vcpu)
>>
>> trace_kvm_aux(vcpu, KVM_TRACE_AUX_RESTORE, KVM_TRACE_AUX_LASX);
>> vcpu->arch.aux_inuse |= KVM_LARCH_LASX | KVM_LARCH_LSX | KVM_LARCH_FPU;
>> - preempt_enable();
>> -
>> return 0;
>> }
>> #endif
>> --
>> 2.39.3
>>
>>
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