linux-kernel - Re: [PATCH V2] LoongArch: Add unaligned access support

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Date:   Mon, 17 Oct 2022 15:37:15 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     Jinyang He <hejinyang@...ngson.cn>
Cc:     Huacai Chen <chenhuacai@...ngson.cn>, loongarch@...ts.linux.dev,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] LoongArch: Add unaligned access support

Hi, Jinyang,

On Mon, Oct 17, 2022 at 12:22 PM Jinyang He <hejinyang@...ngson.cn> wrote:
>
> Hi, Huacai,
>
>
> On 2022/10/17 上午10:23, Huacai Chen wrote:
> > [...]
> > +     default:
> > +             panic("unexpected fd '%d'", fd);
> Due to the optimization of gcc, the panic() is unused actually and leave
> the symbol 'read/write_fpr' in vmlinux. Maybe we can use unreachable() and
>
> always_inline.
Seems impossible, I have tried __always_inline() and BUILD_BUG(), then
BUILD_BUG() is triggered, because the reg-number is not a compile time
constant.

>
> > [...]
> > +
> > +fault:
> > +     /* roll back jump/branch */
> > +     regs->csr_era = origpc;
> > +     regs->regs[1] = origra;
>
> I'm not sure where the csr_era and regs[1] was damaged...
Yes, seems not be damaged.

>
> > [...]
> >
> > +/*
> > + * unsigned long unaligned_read(void *addr, void *value, unsigned long n, bool sign)
> > + *
> > + * a0: addr
> > + * a1: value
> > + * a2: n
> > + * a3: sign
> > + */
> > +SYM_FUNC_START(unaligned_read)
> > +     beqz    a2, 5f
> > +
> > +     li.w    t1, 8
> IMHO we can avoid the constant reg t1.
OK, thanks.

> > +     li.w    t2, 0
> > +
> > +     addi.d  t0, a2, -1
> > +     mul.d   t1, t0, t1
> > +     add.d   a0, a0, t0
> > +
> > +     beq     a3, zero, 2f
> beqz
OK, thanks.

> > +1:   ld.b    t3, a0, 0
> > +     b       3f
> > +
> > +2:   ld.bu   t3, a0, 0
> > +3:   sll.d   t3, t3, t1
> > +     or      t2, t2, t3
> > +     addi.d  t1, t1, -8
> > +     addi.d  a0, a0, -1
> > +     addi.d  a2, a2, -1
> > +     bgt     a2, zero, 2b
> bgtz
> > +4:   st.d    t2, a1, 0
> > +
> > +     move    a0, a2
> > +     jr      ra
> > +
> > +5:   li.w    a0, -EFAULT
> > +     jr      ra
> > +
> > +     fixup_ex 1, 6, 1
> > +     fixup_ex 2, 6, 0
> > +     fixup_ex 4, 6, 0
> > +SYM_FUNC_END(unaligned_read)
> > +
> > +/*
> > + * unsigned long unaligned_write(void *addr, unsigned long value, unsigned long n)
> > + *
> > + * a0: addr
> > + * a1: value
> > + * a2: n
> > + */
> > +SYM_FUNC_START(unaligned_write)
> > +     beqz    a2, 3f
> > +
> > +     li.w    t0, 0
> > +1:   srl.d   t1, a1, t0
> > +2:   st.b    t1, a0, 0
> > +     addi.d  t0, t0, 8
> > +     addi.d  a2, a2, -1
> > +     addi.d  a0, a0, 1
> > +     bgt     a2, zero, 1b
> bgtz
OK, thanks.

> > +
> > +     move    a0, a2
> > +     jr      ra
> > +
> > +3:   li.w    a0, -EFAULT
> > +     jr      ra
> > +
> > +     fixup_ex 2, 4, 1
> > +SYM_FUNC_END(unaligned_write)
>
> Thanks,
>
> Jinyang
>