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Message-ID: <CAAhV-H69F2T-PbewFqch5Sp+xhBP9Lsy74C6yVhp90r4GSYEsQ@mail.gmail.com>
Date: Wed, 28 Jan 2026 12:05:58 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Bibo Mao <maobibo@...ngson.cn>
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
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.
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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