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

[Huacai Chen]

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.


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
>
>