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

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Date:   Mon, 27 Feb 2023 13:19:49 +0100
From:   Johan Almbladh <johan.almbladh@...finetworks.com>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>
Cc:     "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        linux-kernel@...r.kernel.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "paulburton@...nel.org" <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 Thu, Feb 23, 2023 at 11:32 AM Jiaxun Yang <jiaxun.yang@...goat.com> wrote:
> >> --- 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?
>
> Yes, R4400 errata is basically a reduced version of R4000 one.
> They managed to fix some parts of the issue but not all.

Ok.

> >> --- 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 */

For context, please specify which workaround this comment refers to:
"workaround" -> "R4000 workaround".

> > 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
>
> It’s common practice to run 32-bit kernel on R4000 based systems to save some memory :-)

Ok, I understand.

Looks good! I have run the test_bpf.ko test suite on MIPS and MIPS64
in QEMU with and without the workarounds enabled.

With above comment fix:
Acked-by: Johan Almbladh <johan.almbladh@...finetworks.com>