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

From: Jisheng Zhang <jszhang@kernel.org>
To: Andreas Schwab <schwab@suse.de>
Cc: Arnd Bergmann <arnd@arndb.de>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/4] riscv: uaccess: use input constraints for ptr of __put_user
Date: Wed, 26 Jun 2024 21:54:07 +0800	[thread overview]
Message-ID: <Znwdf2c1pYetrsHe@xhacker> (raw)
In-Reply-To: <mvma5j7omh1.fsf@suse.de>

On Wed, Jun 26, 2024 at 03:35:54PM +0200, Andreas Schwab wrote:
> 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.

Hi Andreas,

Your information is clearly received. What confused me is:

why x86 and arm64 don't put the "addr" of __put_user into output
constraints? Especially the following comments, why this is "read"
from memory?

 * Tell gcc we read from memory instead of writing: this is because
 * we do not write to any memory gcc knows about, so there are no
 * aliasing issues.

can you please kindly help me understand the tricky points here?

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

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