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Message-ID: <b9f311be-88f6-ffca-fc8e-70bec2cf7a75@loongson.cn>
Date: Tue, 3 Feb 2026 14:45:21 +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 v3 4/4] LoongArch: KVM: Add FPU delay load support
On 2026/2/3 下午12:15, Huacai Chen wrote:
> Hi, Bibo,
>
> On Tue, Feb 3, 2026 at 11:31 AM 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, there will
>> be unnecessary FPU exception, load and store process. Here FPU is
>> delay load until guest enter entry.
> Calling LSX/LASX as FPU is a little strange, but somewhat reasonable.
> Calling LBT as FPU is very strange. So I still like the V1 logic.
yeap, LBT can use another different BIT and separate with FPU. It is
actually normal use one bit + fpu type variant to represent different
different FPU load requirement, such as
TIF_FOREIGN_FPSTATE/TIF_NEED_FPU_LOAD on other architectures.
I think it is better to put int fpu_load_type in structure loongarch_fpu.
And there will be another optimization to avoid load FPU again if FPU HW
is owned by current thread/vCPU, that will add last_cpu int type in
structure loongarch_fpu also.
Regards
Bibo Mao
>
> If you insist on this version, please rename KVM_REQ_FPU_LOAD to
> KVM_REQ_AUX_LOAD and rename fpu_load_type to aux_type, which is
> similar to aux_inuse.
>
> Huacai
>
>>
>> Signed-off-by: Bibo Mao <maobibo@...ngson.cn>
>> ---
>> arch/loongarch/include/asm/kvm_host.h | 2 ++
>> arch/loongarch/kvm/exit.c | 21 ++++++++++-----
>> arch/loongarch/kvm/vcpu.c | 37 ++++++++++++++++++---------
>> 3 files changed, 41 insertions(+), 19 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;
>> /* 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 65ec10a7245a..62403c7c6f9a 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;
>> }
>> @@ -822,8 +827,10 @@ static int kvm_handle_lbt_disabled(struct kvm_vcpu *vcpu, int ecode)
>> {
>> if (!kvm_guest_has_lbt(&vcpu->arch))
>> kvm_queue_exception(vcpu, EXCCODE_INE, 0);
>> - else
>> - kvm_own_lbt(vcpu);
>> + else {
>> + vcpu->arch.fpu_load_type = KVM_LARCH_LBT;
>> + 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 995461d724b5..d05fe6c8f456 100644
>> --- a/arch/loongarch/kvm/vcpu.c
>> +++ b/arch/loongarch/kvm/vcpu.c
>> @@ -232,6 +232,31 @@ 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;
>> +
>> + case KVM_LARCH_LBT:
>> + kvm_own_lbt(vcpu);
>> + break;
>> +
>> + default:
>> + break;
>> + }
>> +
>> + vcpu->arch.fpu_load_type = 0;
>> + }
>> }
>>
>> /*
>> @@ -1286,13 +1311,11 @@ int kvm_arch_vcpu_ioctl_set_fpu(struct kvm_vcpu *vcpu, struct kvm_fpu *fpu)
>> #ifdef CONFIG_CPU_HAS_LBT
>> int kvm_own_lbt(struct kvm_vcpu *vcpu)
>> {
>> - preempt_disable();
>> if (!(vcpu->arch.aux_inuse & KVM_LARCH_LBT)) {
>> set_csr_euen(CSR_EUEN_LBTEN);
>> _restore_lbt(&vcpu->arch.lbt);
>> vcpu->arch.aux_inuse |= KVM_LARCH_LBT;
>> }
>> - preempt_enable();
>>
>> return 0;
>> }
>> @@ -1335,8 +1358,6 @@ 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
>> @@ -1347,16 +1368,12 @@ 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);
>> @@ -1378,7 +1395,6 @@ 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;
>> }
>> @@ -1388,8 +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)) {
>> @@ -1411,7 +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;
>> }
>> --
>> 2.39.3
>>
>>
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