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From:   Guo Ren <guoren@kernel.org>
Date:   Wed, 23 Sep 2020 10:37:31 +0800
Message-ID: <CAJF2gTSU4e4yU63z1q502SeuTf2m2BKaD0yZ0deFj0TkiVupFg@mail.gmail.com>
Subject: Re: [PATCH] csky: Fix a size determination in gpr_get()
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Zhenzhong Duan <zhenzhong.duan@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-csky@vger.kernel.org, Oleg Nesterov <oleg@redhat.com>
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On Wed, Sep 23, 2020 at 8:23 AM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Wed, Sep 23, 2020 at 08:03:20AM +0800, Guo Ren wrote:
> > Thx Duan,
> >
> > Acked-by: Guo Ren <guoren@kernel.org>
> >
> > Hi AI,
> >
> > I found the broken commit still has a question:
> >
> > > commit dcad7854fcce6a2d49b6a3ead5bbefeff047e559
> > > Author: Al Viro <viro@zeniv.linux.org.uk>
> > > Date:   Tue Jun 16 15:28:29 2020 -0400
> >
> > >    csky: switch to ->regset_get()
> >
> > >    NB: WTF "- what the fuck :(" is fpregs_get() playing at???
> > The fpregs_get() is for REGSET_FPR regset used by ptrace (gdb) and all
> > fp regs are stored in threads' context.
> > So, WTF question for?
>
> The part under
> #if defined(CONFIG_CPU_HAS_FPUV2) && !defined(CONFIG_CPU_HAS_VDSP)
>
> What's going on there?  The mapping is really weird - assuming
> you had v0..v31 in the first 32 elements of regs->vr[], you
> end up with
>
> v0 v1 v2 v3 v2 v3 v6 v7 v4 v5 v10 v11 v6 v7 v14 v15
> v8 v9 v18 v19 v10 v11 v22 v23 v12 v13 v26 v27 v14 v15 v30 v31
>
> in the beginning of the output.  Assuming it is the intended
> behaviour, it's probably worth some comments...
FPU & VDSP use the same regs. 32 FPU regs' width is 64b and 16 VDSP
regs' width is 128b.

vr[0], vr[1] = fp[0] & vr[0] vr[1], vr[2], vr[3] = vdsp reg[0]
...
vr[60], vr[61] = fp[15] & vr[60] vr[61], vr[62], vr[63] = vdsp reg[15]
vr[64], vr[65] = fp[16]
vr[66], vr[67] = fp[17]
...
vr[94], vr[95] = fp[31]

Yeah, this is confusing and I'll add a comment later.


--
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/