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

[Huacai Chen]

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
power consumption of LASX is significantly more than FPU, which is a
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
> >
> >