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Message-ID: <CAAhV-H6v2oVe38HX7k_wvysUx4nyz6pbfUOU7wiaJO+1A3ASJw@mail.gmail.com>
Date: Tue, 3 Feb 2026 15:34:13 +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 v3 4/4] LoongArch: KVM: Add FPU delay load support
On Tue, Feb 3, 2026 at 2:48 PM Bibo Mao <maobibo@...ngson.cn> wrote:
>
>
>
> 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.
Then why not consider this?
Huacai
> >
> > 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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