RE: [PATCH 2/4] riscv: uaccess: use input constraints for ptr of __put_user

From: David Laight <David.Laight@ACULAB.COM>
To: 'Arnd Bergmann' <arnd@arndb.de>,
	Jisheng Zhang <jszhang@kernel.org>,
	Andreas Schwab <schwab@suse.de>
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	"linux-riscv@lists.infradead.org"
	<linux-riscv@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH 2/4] riscv: uaccess: use input constraints for ptr of __put_user
Date: Fri, 28 Jun 2024 15:36:17 +0000	[thread overview]
Message-ID: <b4d15c17ed684f5ab3cf89fda9f73618@AcuMS.aculab.com> (raw)
In-Reply-To: <dcf9574a-0f1b-4131-befd-39f47d4f9002@app.fastmail.com>

From: Arnd Bergmann
> Sent: 26 June 2024 15:25
...
> If you just copy from the arm64 version that uses an
> "r"(address) constraint instead of the "m"(*address)
> version, it should be fine for any user space access.

Arm certainly has 'reg+offset' addressing and I'd have thought
the RISC-V would have it as well.

I'd guess that the compiler also knows when the offset is too big.

Probably noticeable when code is accessing structures in user memory.

OTOH I can't remember if "m" implies a memory clobber?
For user copies the memory clobber isn't needed and not having it
may well allow better instruction scheduling.

	David

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