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

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Message-ID: <cbbc32d4bb0f445197eb5d84d796595c@AcuMS.aculab.com>
Date:   Tue, 18 Oct 2022 07:48:04 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Huacai Chen' <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>
CC:     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

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.

	David

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