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Subject: Re: [PATCH 2/2] MIPS: ebpf jit: Implement R4000 workarounds
From:   Jiaxun Yang <jiaxun.yang@flygoat.com>
In-Reply-To: <CAM1=_QTDkYJANgxYwkgPZB+hUX6Rr_Pvnn7cFwSJFHQtLrpQMA@mail.gmail.com>
Date:   Thu, 23 Feb 2023 10:31:54 +0000
Cc:     "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
        linux-kernel@vger.kernel.org,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        "paulburton@kernel.org" <paulburton@kernel.org>,
        bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>
Content-Transfer-Encoding: quoted-printable
Message-Id: <70C80F6D-A727-48FD-A767-A2CA54AA7C1E@flygoat.com>
References: <20230222161222.11879-1-jiaxun.yang@flygoat.com>
 <20230222161222.11879-3-jiaxun.yang@flygoat.com>
 <CAM1=_QTDkYJANgxYwkgPZB+hUX6Rr_Pvnn7cFwSJFHQtLrpQMA@mail.gmail.com>
To:     Johan Almbladh <johan.almbladh@anyfinetworks.com>
Precedence: bulk


> 2023=E5=B9=B42=E6=9C=8823=E6=97=A5 10:22=EF=BC=8CJohan Almbladh =
<johan.almbladh@anyfinetworks.com> =E5=86=99=E9=81=93=EF=BC=9A
>=20
> On Wed, Feb 22, 2023 at 5:12 PM Jiaxun Yang <jiaxun.yang@flygoat.com> =
wrote:
>>=20
>> 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
>>=20
>> "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."
>>=20
>> We just emit a mfhi $0 to ensure the operation is completed after
>> every multiplication instruction accorading to workaround suggestion
>> in the document.
>>=20
>> Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.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(-)
>>=20
>> 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
>=20
> 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.

>=20
>> +       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);
>>                }
>>=20
>>                /* hi(dst) =3D 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);
>>        }
>=20
> R4000 is a 64-bit CPU, so the 32-bit JIT implementation will not be
> used. =46rom the Makefile:
>=20
> ifeq ($(CONFIG_32BIT),y)
>        obj-$(CONFIG_BPF_JIT) +=3D bpf_jit_comp32.o
> else
>        obj-$(CONFIG_BPF_JIT) +=3D bpf_jit_comp64.o
> endif

It=E2=80=99s common practice to run 32-bit kernel on R4000 based systems =
to save some memory :-)

Thanks.
- Jiaxun

>=20
>>=20
>> 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 =3D dst / src */
>> --
>> 2.37.1 (Apple Git-137.1)