linux-kernel - Re: [PATCH 2/2] MIPS: ebpf jit: Implement R4000 workarounds

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Date:   Thu, 23 Feb 2023 11:22:43 +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>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH 2/2] MIPS: ebpf jit: Implement R4000 workarounds

On Wed, Feb 22, 2023 at 5:12 PM Jiaxun Yang <jiaxun.yang@...goat.com> wrote:
>
> For R4000 erratas around multiplication and division instructions,
> as our use of those instructions are always followed by mflo/mfhi
> instructions, the only issue we need care is
>
> "MIPS R4000PC/SC Errata, Processor Revision 2.2 and 3.0" Errata 28:
> "A double-word or a variable shift may give an incorrect result if
> executed while an integer multiplication is in progress."
>
> We just emit a mfhi $0 to ensure the operation is completed after
> every multiplication instruction accorading to workaround suggestion
> in the document.
>
> Signed-off-by: Jiaxun Yang <jiaxun.yang@...goat.com>
> ---
>  arch/mips/Kconfig              | 4 +---
>  arch/mips/net/bpf_jit_comp32.c | 4 ++++
>  arch/mips/net/bpf_jit_comp64.c | 3 +++
>  3 files changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> index df0910e3895c..5ea07c833c5b 100644
> --- a/arch/mips/Kconfig
> +++ b/arch/mips/Kconfig
> @@ -63,9 +63,7 @@ config MIPS
>         select HAVE_DEBUG_STACKOVERFLOW
>         select HAVE_DMA_CONTIGUOUS
>         select HAVE_DYNAMIC_FTRACE
> -       select HAVE_EBPF_JIT if !CPU_MICROMIPS && \
> -                               !CPU_R4000_WORKAROUNDS && \
> -                               !CPU_R4400_WORKAROUNDS

Is the R4400 errata also covered by this workaround?

> +       select HAVE_EBPF_JIT if !CPU_MICROMIPS
>         select HAVE_EXIT_THREAD
>         select HAVE_FAST_GUP
>         select HAVE_FTRACE_MCOUNT_RECORD
> diff --git a/arch/mips/net/bpf_jit_comp32.c b/arch/mips/net/bpf_jit_comp32.c
> index ace5db3fbd17..fee334544d2f 100644
> --- a/arch/mips/net/bpf_jit_comp32.c
> +++ b/arch/mips/net/bpf_jit_comp32.c
> @@ -446,6 +446,9 @@ static void emit_mul_i64(struct jit_context *ctx, const u8 dst[], s32 imm)
>                 } else {
>                         emit(ctx, multu, hi(dst), src);
>                         emit(ctx, mflo, hi(dst));
> +                       /* Ensure multiplication is completed */
> +                       if (IS_ENABLED(CONFIG_CPU_R4000_WORKAROUNDS))
> +                               emit(ctx, mfhi, MIPS_R_ZERO);
>                 }
>
>                 /* hi(dst) = hi(dst) - lo(dst) */
> @@ -504,6 +507,7 @@ static void emit_mul_r64(struct jit_context *ctx,
>         } else {
>                 emit(ctx, multu, lo(dst), lo(src));
>                 emit(ctx, mflo, lo(dst));
> +               /* No need for workaround because we have this mfhi */
>                 emit(ctx, mfhi, tmp);
>         }

R4000 is a 64-bit CPU, so the 32-bit JIT implementation will not be
used. From the Makefile:

ifeq ($(CONFIG_32BIT),y)
        obj-$(CONFIG_BPF_JIT) += bpf_jit_comp32.o
else
        obj-$(CONFIG_BPF_JIT) += bpf_jit_comp64.o
endif

>
> diff --git a/arch/mips/net/bpf_jit_comp64.c b/arch/mips/net/bpf_jit_comp64.c
> index 0e7c1bdcf914..5f5a93f997bc 100644
> --- a/arch/mips/net/bpf_jit_comp64.c
> +++ b/arch/mips/net/bpf_jit_comp64.c
> @@ -228,6 +228,9 @@ static void emit_alu_r64(struct jit_context *ctx, u8 dst, u8 src, u8 op)
>                 } else {
>                         emit(ctx, dmultu, dst, src);
>                         emit(ctx, mflo, dst);
> +                       /* Ensure multiplication is completed */
> +                       if (IS_ENABLED(CONFIG_CPU_R4000_WORKAROUNDS))
> +                               emit(ctx, mfhi, MIPS_R_ZERO);
>                 }
>                 break;
>         /* dst = dst / src */
> --
> 2.37.1 (Apple Git-137.1)
>