linux-kernel - Re: [PATCH 2/4] riscv: uaccess: use input constraints for ptr of __put

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Date: Thu, 27 Jun 2024 08:46:19 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Jisheng Zhang" <jszhang@...nel.org>
Cc: "Andreas Schwab" <schwab@...e.de>,
 "Paul Walmsley" <paul.walmsley@...ive.com>,
 "Palmer Dabbelt" <palmer@...belt.com>, "Albert Ou" <aou@...s.berkeley.edu>,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] riscv: uaccess: use input constraints for ptr of __put_user

On Wed, Jun 26, 2024, at 18:02, Jisheng Zhang wrote:
>
> "m" version is better than "r", usually can save one
> instruction.
> I will try to combine other constraints with "r" to
> see whether we can still generate the sd with offset
> instruction. If can't, seems sticking with "m" and keeping
> output constraints is better

Ah, I see.

> You only mention "user space access", so just curious, does
> arm64 version still correctly work with below __put_kernel_nofault()
> example?

No, I think the example I gave would break for both x86 and arm64
without adding an output constraint.

My main concern about using an input constraint was that it
doesn't match what the code does. Maybe there is a way to
make it use the correct "=m" output when CONFIG_CC_HAS_ASM_GOTO_OUTPUT
is set but use either "r" or "m" inputs on older gcc releases.

After gcc-11 becomes the minimum in a few years, the hack can
be removed.

     Arnd