linux-kernel - Re: [PATCH 1/2] MIPS: ebpf jit: Implement DADDI workarounds

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Date:   Thu, 23 Feb 2023 11:10:07 +0100
From:   Johan Almbladh <johan.almbladh@...finetworks.com>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>
Cc:     linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        tsbogend@...ha.franken.de, paulburton@...nel.org,
        bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH 1/2] MIPS: ebpf jit: Implement DADDI workarounds

On Wed, Feb 22, 2023 at 5:12 PM Jiaxun Yang <jiaxun.yang@...goat.com> wrote:
>
> For DADDI errata we just workaround by disable immediate operation
> for BPF_ADD / BPF_SUB to avoid generation of DADDIU.

Good, this is an elegant solution to trigger fallback to the
register-only operation. Does the DADDI errata only affect the DADDIU,
not DADDI?

>
> All other use cases in JIT won't cause overflow thus they are all safe.

There are quite a few other places where DADDIU is emitted. How do you
know those are safe? I am interested in your reasoning here, as I
don't know what would be safe and not.

>
> Signed-off-by: Jiaxun Yang <jiaxun.yang@...goat.com>
> ---
>  arch/mips/Kconfig            | 1 -
>  arch/mips/net/bpf_jit_comp.c | 8 ++++++++
>  2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> index 37072e15b263..df0910e3895c 100644
> --- a/arch/mips/Kconfig
> +++ b/arch/mips/Kconfig
> @@ -64,7 +64,6 @@ config MIPS
>         select HAVE_DMA_CONTIGUOUS
>         select HAVE_DYNAMIC_FTRACE
>         select HAVE_EBPF_JIT if !CPU_MICROMIPS && \
> -                               !CPU_DADDI_WORKAROUNDS && \
>                                 !CPU_R4000_WORKAROUNDS && \
>                                 !CPU_R4400_WORKAROUNDS
>         select HAVE_EXIT_THREAD
> diff --git a/arch/mips/net/bpf_jit_comp.c b/arch/mips/net/bpf_jit_comp.c
> index b17130d510d4..7110a6687f7a 100644
> --- a/arch/mips/net/bpf_jit_comp.c
> +++ b/arch/mips/net/bpf_jit_comp.c
> @@ -218,9 +218,17 @@ bool valid_alu_i(u8 op, s32 imm)
>                 /* All legal eBPF values are valid */
>                 return true;
>         case BPF_ADD:
> +#ifdef CONFIG_64BIT

DADDI/DADDIU are only available on 64-bit CPUs, so the errata would
only be applicable to that. No need for the CONFIG_64BIT conditional.

> +               if (IS_ENABLED(CONFIG_CPU_DADDI_WORKAROUNDS))
> +                       return false;
> +#endif
>                 /* imm must be 16 bits */
>                 return imm >= -0x8000 && imm <= 0x7fff;
>         case BPF_SUB:
> +#ifdef CONFIG_64BIT
> +               if (IS_ENABLED(CONFIG_CPU_DADDI_WORKAROUNDS))
> +                       return false;
> +#endif
>                 /* -imm must be 16 bits */
>                 return imm >= -0x7fff && imm <= 0x8000;
>         case BPF_AND:
> --
> 2.37.1 (Apple Git-137.1)
>