Re: [PATCH V2] LoongArch: Add unaligned access support

[WANG Xuerui]

On 2022/10/18 10:24, Huacai Chen wrote:
> Hi, David,
>
> On Mon, Oct 17, 2022 at 8:58 PM David Laight <David.Laight@xxxxxxxxxx> wrote:
> > From: Huacai Chen
> > > Sent: 17 October 2022 03:24
> > >
> > > Loongson-2 series (Loongson-2K500, Loongson-2K1000) don't support
> > > unaligned access in hardware, while Loongson-3 series (Loongson-3A5000,
> > > Loongson-3C5000) are configurable whether support unaligned access in
> > > hardware. This patch add unaligned access emulation for those LoongArch
> > > processors without hardware support.
> > ...
> > > +     /*
> > > +      * This load never faults.
> > > +      */
> > > +     __get_inst(&insn.word, pc, user);
> >
> > On what basis does it never fault?
> > Any user access can fault.
> > If nothing else another thread of the process can unmap
> > the page.
> Yes, this can happen, since __get_inst() handles fault, we can just
> remove the comment.
>
> > > +     if (user && !access_ok(addr, 8))
> > > +             goto sigbus;
> >
> > Surely that is technically wrong - a two or four byte
> > access is valid right at the end of valid user addreeses.
> Yes, this check should be moved to each case.
>
> > > +
> > > +     if (insn.reg2i12_format.opcode == ldd_op ||
> > > +             insn.reg2i14_format.opcode == ldptrd_op ||
> > > +             insn.reg3_format.opcode == ldxd_op) {
> > > +             res = unaligned_read(addr, &value, 8, 1);
> >
> > That is the most horrid indentation of long lines I've
> > ever seen.
> > I'd also guess you can common up some of this code
> > by looking at the instruction field that include the
> > transfer width.
> >
> > The long elsif list will generate horrid code.
> > But maybe since you've just taken a fault it really
> > doesn't matter.
> > Indeed just emulating in C using byte accesses
> > it probably fine.
> I want to keep the assembly, because we can use more efficient methods
> with the upcoming alternative mechanism.

What about my more structured approach in another reply that avoids the 
huge else-if conditions? Both the terrible line wraps and codegen could 
be avoided.

--
WANG "xen0n" Xuerui

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