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

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Message-ID: <CAAhV-H42_dNyfYHDpdmPM=kG=VTF8xtPHMn-ojWHNkLnTNVDiQ@mail.gmail.com>
Date:   Tue, 18 Oct 2022 17:36:45 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     WANG Xuerui <kernel@...0n.name>,
        Huacai Chen <chenhuacai@...ngson.cn>,
        "loongarch@...ts.linux.dev" <loongarch@...ts.linux.dev>,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Guo Ren <guoren@...nel.org>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] LoongArch: Add unaligned access support

On Tue, Oct 18, 2022 at 3:48 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Huacai Chen
> > Sent: 18 October 2022 08:33
> ...
> > > 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.
> ...
> > OK, let me try.
>
> I suspect you can mask out some 'operand size' bits from the
> instructions - instead of checking each opcode.
>
> I'm also pretty sure you can't assume the FP register are live.
> If a read from userspace faults then there can be a full
> process switch - so by the time you try to write to the
> FP registers they no longer belong to the current process.
>
> It might be safer and simpler to just enforce the FP
> registers be saved and then act on the save area.
> I'd guess they get restored in the 'return to userspace'
> code.
Good catch, will be fixed in V4.

Huacai
>
>         David
>
> -
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