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

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Message-ID: <mvma5j7omh1.fsf@suse.de>
Date: Wed, 26 Jun 2024 15:35:54 +0200
From: Andreas Schwab <schwab@...e.de>
To: Jisheng Zhang <jszhang@...nel.org>
Cc: Arnd Bergmann <arnd@...db.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 Jun 26 2024, Jisheng Zhang wrote:

> no output constraints either. It just uses "r" input constraints to tell
> gcc to read the store address into one proper GP reg.

Again, this is backwards.  Being an input operand means the asm is using
this operand as an input to the instructions.  The compiler needs to
arrange to put the value in the allocated operand location according to
the constraint.

-- 
Andreas Schwab, SUSE Labs, schwab@...e.de
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."