Re: [RFC v3 1/2] target/loongarch: Add loongson binary translation feature

["Jiaxun Yang" <jiaxun.yang@flygoat.com>]

在2024年5月30日五月 上午7:49，Bibo Mao写道：
> Loongson Binary Translation (LBT) is used to accelerate binary
> translation, which contains 4 scratch registers (scr0 to scr3), x86/ARM
> eflags (eflags) and x87 fpu stack pointer (ftop).
>
> Now LBT feature is added in kvm mode, not supported in TCG mode since
> it is not emulated. Feature variable lbt is added with OnOffAuto type,
> If lbt feature is not supported with KVM host, it reports error if there
> is lbt=on command line.
>
> If there is no any command line about lbt parameter, it checks whether
> KVM host supports lbt feature and set the corresponding value in cpucfg.
>
> Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Hi Bibo,

I was going across recent LoongArch changes and this comes into my attention: 

> ---
>  target/loongarch/cpu.c                | 53 +++++++++++++++++++++++++++
>  target/loongarch/cpu.h                |  6 +++
>  target/loongarch/kvm/kvm.c            | 26 +++++++++++++
>  target/loongarch/kvm/kvm_loongarch.h  | 16 ++++++++
>  target/loongarch/loongarch-qmp-cmds.c |  2 +-
>  5 files changed, 102 insertions(+), 1 deletion(-)
>
> diff --git a/target/loongarch/cpu.c b/target/loongarch/cpu.c
> index b5c1ec94af..14265b6667 100644
> --- a/target/loongarch/cpu.c
> +++ b/target/loongarch/cpu.c
> @@ -571,6 +571,30 @@ static void loongarch_cpu_disas_set_info(CPUState 
> *s, disassemble_info *info)
>      info->print_insn = print_insn_loongarch;
>  }
> 
> +static void loongarch_cpu_check_lbt(CPUState *cs, Error **errp)
> +{
> +    CPULoongArchState *env = cpu_env(cs);
> +    LoongArchCPU *cpu = LOONGARCH_CPU(cs);
> +    bool kvm_supported;
> +
> +    kvm_supported = kvm_feature_supported(cs, LOONGARCH_FEATURE_LBT);

IMHO if there is no global states that should be saved/restored VM wise,
this should be handled at per CPU level, preferably with CPUCFG flags hint.

We should minimize non-privilege KVM feature bits to prevent hindering
asymmetry ISA system.

Thanks
- Jiaxun

-- 
- Jiaxun