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

[Jiaxun Yang]

> 2023年2月23日 10:22，Johan Almbladh <johan.almbladh@xxxxxxxxxxxxxxxxx> 写道：
> 
> On Wed, Feb 22, 2023 at 5:12 PM Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx> 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@xxxxxxxxxxx>
>> ---
>> 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?

Yes, R4400 errata is basically a reduced version of R4000 one.
They managed to fix some parts of the issue but not all.

> 
>> +       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

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

Thanks.
- Jiaxun

> 
>> 
>> 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)