Re: [PATCH v2 0/6] LoongArch: KVM: Set max VM supported FPU type with FPU exception

[Huacai Chen]

On Thu, Apr 9, 2026 at 8:14 PM Bibo Mao <maobibo@xxxxxxxxxxx> wrote:
>
>
>
> On 2026/4/8 下午3:47, Huacai Chen wrote:
> > On Wed, Apr 8, 2026 at 9:04 AM Bibo Mao <maobibo@xxxxxxxxxxx> wrote:
> >>
> >>
> >>
> >> On 2026/4/7 下午8:41, Huacai Chen wrote:
> >>> On Mon, Mar 30, 2026 at 6:00 PM Huacai Chen <chenhuacai@xxxxxxxxxx> wrote:
> >>>>
> >>>> Hi, Bibo,
> >>>>
> >>>>
> >>>> On Mon, Mar 30, 2026 at 11:58 AM Bibo Mao <maobibo@xxxxxxxxxxx> wrote:
> >>>>>
> >>>>> With FPU save and restore flow, the cost is the same with different
> >>>>> FPU width 8/16/32 bytes, whatever from CPU cycle and cache line impaction.
> >>>>>
> >>>>> Here is to enable FPU with max VM supported type, for example if
> >>>>> VM supports LASX instrction, enable FPU with LASX type even with FPU
> >>>>> exeception. So it can avoid possible LSX/LASX exception in future.
> >>>>>
> >>>>> With context switch microbench which may touch FPU and LASX, there is 9%
> >>>>> improvement when halt_poll_ns is disabled. The command is
> >>>>> "./context --test=pipe" and source code located at:
> >>>>>     https://github.com/bibo-mao/context_switch/blob/main/context.c
> >>>>>
> >>>>> Original     Wih patch      improvement
> >>>>> 75232        82440          9%
> >>>>>
> >>>>> Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx>
> >>>>> ---
> >>>>> v1 ... v2:
> >>>>>     1. Enable FPU with max VM supported FPU type, rather than max used type.
> >>>>>     2. Add new request bit KVM_REQ_LBT_LOAD for LBT restore
> >>>>>     3. Rename KVM_REQ_AUX_LOAD with KVM_REQ_FPU_LOAD
> >>>>>     3. Remove aux_ldtype and KVM_LARCH_LSX/KVM_LARCH_LSX
> >>>>>     4. Remove middle FPU state handling in kvm_own_lsx() and kvm_own_lasx(),
> >>>>>        directly enable LSX or LASX from FPU none state.
> >>>>> ---
> >>>>> Bibo Mao (6):
> >>>>>     LoongArch: KVM: Add separate KVM_REQ_LBT_LOAD request bit
> >>>>>     LoongArch: KVM: Enable FPU with max VM supported FPU type
> >>>>>     LoongArch: KVM: Rename KVM_REQ_AUX_LOAD with KVM_REQ_FPU_LOAD
> >>>> Patch-1 add KVM_REQ_LBT_LOAD, then KVM_REQ_AUX_LOAD is only for FPU,
> >>>> so I think Patch-3 should be squashed into Patch-1.
> >>>>
> >>>>>     LoongArch: KVM: Remove some middle FPU states
> >>>>>     LoongArch: KVM: Use vm_guest_has_fpu API in kvm_lose_fpu()
> >>>>>     LoongArch: KVM: Remove KVM_LARCH_LASX and KVM_LARCH_LSX
> >>>> Patch-5 remove the consumer side of KVM_LARCH_LASX / KVM_LARCH_LSX and
> >>>> Patch-6 remove the provider side of KVM_LARCH_LASX / KVM_LARCH_LSX, so
> >>>> I think Patch-6 should be squashed into Patch-5, too.
> >>> And could you please test the power consumption where there are many
> >>> VMs that only use FPU rather than LSX/LASX? As far as I know, the
> >> Hi Huacai,
> >>
> >> Thanks for reviewing this patch
> >> Could you help me to test the power consumption with this patch?
> >>> power consumption of LASX is significantly more than FPU, which is a
> >> Could the describe the detail scenery wheree LASX instruction is
> >> frequently used by application or LASX  EUEN enabled?
> > The purpose is to evaluate power consumption before and after this series.
> >
> > So we can run 10 VMs (suppose there are 4 cores), and there is a
> > workload run in every VM, the workload only uses FPU, no LSX and LASX.
> > Before this series, KVM only save/restore FPU context; after this
> > series, KVM will save/restore LASX context.
> In theory there should be such test, only that it requires special Power
> Management team and testbed. Even if there is such team, maybe they care
> power consumption about DDR and screen, do not want do such experiments
> for this patch.
>
> If someone is volunteer to do this, I do not object, also I want to get
> the result. Just from my guess, there should be no much difference since
> FPU context switch is much smaller than the whole workload, else the
> workload is just FPU context switch.
Any updates?


Huacai
>
> Regards
> Bibo Mao
>
> >
> > If the power consumptions are similar, then this series is perfect.
> >
> > Huacai
> >
> >>
> >> Regards
> >> Bibo Mao
> >>> little similar to AVX512 (Linus said it is a power virus).
> >>>
> >>> Huacai
> >>>
> >>>>
> >>>>
> >>>> Huacai
> >>>>
> >>>>>
> >>>>>    arch/loongarch/include/asm/kvm_host.h |  6 +--
> >>>>>    arch/loongarch/kvm/exit.c             | 21 +++-----
> >>>>>    arch/loongarch/kvm/vcpu.c             | 78 ++++++++-------------------
> >>>>>    3 files changed, 30 insertions(+), 75 deletions(-)
> >>>>>
> >>>>>
> >>>>> base-commit: 7aaa8047eafd0bd628065b15757d9b48c5f9c07d
> >>>>> --
> >>>>> 2.39.3
> >>>>>
> >>>>>
> >>
> >>
>
>