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

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Message-ID: <ZnwTwnSsnZ8Td9GZ@xhacker>
Date: Wed, 26 Jun 2024 21:12:34 +0800
From: Jisheng Zhang <jszhang@...nel.org>
To: Andreas Schwab <schwab@...e.de>
Cc: 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 03:12:50PM +0200, Andreas Schwab wrote:
> On Jun 25 2024, Jisheng Zhang wrote:
> 
> > I believe the output constraints "=m" is not necessary, because
> > the instruction itself is "write", we don't need the compiler
> > to "write" for us.
> 
> No, this is backwards.  Being an output operand means that the *asm* is
> writing to it, and the compiler can read the value from there afterwards
> (and the previous value is dead before the asm).

Hi Andreas,

I compared tens of __put_user() caller's generated code between orig
version and patched version, they are the same. Sure maybe this is
not enough. 

But your explanation can be applied to x86 and arm64 __put_user()
implementations, asm is also writing, then why there's no output
constraints there?(see the other two emails)? Could you please help
me to understand the tricky points?

Thanks in advance