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

[Bibo Mao]

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?

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