Re: [PATCH v3 4/4] LoongArch: KVM: Add FPU delay load support
From: Bibo Mao
Date: Tue Feb 03 2026 - 04:32:35 EST
On 2026/2/3 下午5:17, Huacai Chen wrote:
On Tue, Feb 3, 2026 at 4:59 PM Bibo Mao <maobibo@xxxxxxxxxxx> wrote:I do not say easy fpu should be used. The problem is that for lazy FPU exception, if FPU HW register is the same with SW FPU state though FPU is disabled, can the FPU load be skipped and only enable FPU by EUEN register?
aux_type is renamed from fpu_load_type, so where fpu_load_type is,
On 2026/2/3 下午4:50, Huacai Chen wrote:
On Tue, Feb 3, 2026 at 3:51 PM Bibo Mao <maobibo@xxxxxxxxxxx> wrote:where aux_type is put for kernel/kvm? Put it in thread structure with
Yes, but two structures don't block us from using KVM_REQ_AUX_LOAD and
On 2026/2/3 下午3:34, Huacai Chen wrote:
On Tue, Feb 3, 2026 at 2:48 PM Bibo Mao <maobibo@xxxxxxxxxxx> wrote:this can work now. However there is two different structure struct
Then why not consider this?
On 2026/2/3 下午12:15, Huacai Chen wrote:
Hi, Bibo,yeap, LBT can use another different BIT and separate with FPU. It is
On Tue, Feb 3, 2026 at 11:31 AM Bibo Mao <maobibo@xxxxxxxxxxx> wrote:
Calling LSX/LASX as FPU is a little strange, but somewhat reasonable.
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 LBT as FPU is very strange. So I still like the V1 logic.
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.
loongarch_fpu and struct loongarch_lbt.
aux_type to abstract both FPU and LBT, which is similar to aux_inuse.
1. If kernel wants to use late FPU load, new element fpu_load_type can
be added in struct loongarch_fpu for both user app/KVM.
kernel late FPU load and vcpu.arch with KVM late FPU load?
then where aux_type is.
I investigate nothing, including MIPS. Other architectures may give usIf vCPU using FPU is preempted by kernel thread and kernel thread doesBoth loongarch_fpu and loongarch_lbt are register copies, so adding
2. With further optimization, FPU HW can own by user app/kernel/KVM,
there will be another last_cpu int type added in struct loongarch_fpu.
fpu_load_type/last_cpu is not a good idea.
not use FPU, HW FPU is the same with SW FPU state, HW FPU load can be
skipped.
BTW do you ever investigate FPU load/save process on other general
architectures except MIPS?
some inspiration, but that doesn't mean we should copy them, no matter
X86 or MIPS.
X86 introduced lazy fpu, then others also use lazy fpu; but now X86
have switched to eager fpu, others should also do the same?
The trace function is good suggestion, so I accept and post the second version. centralized FPU/LSX/LASX is because kvm_late_check_requests() is critical hot path, however FPU load is not, it is not necessary to add three cold kvm_check_request().
On the other hand, when you use separate FPU/LSX/LASX, I only mention
the trace functions. Then you changed to centralized FPU/LSX/LASX/LBT.
Then I suggest you improve centralized FPU/LSX/LASX/LBT, you changedNow the LBT and FPU uses different structure to save and restore, FPU and LBT is separated already, however FPU/LSX/LASX uses the same structure loongarch_fpu.
to separate FPU/LBT again, where is the end?
Regards
Bibo Mao
Huacai
Regards
Bibo Mao
Huacai
Regards
Bibo Mao
Regards
Bibo Mao
Huacai
Huacai
Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx>
---
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