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

[Huacai Chen]

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.

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
> >>>
> >>>
>
>