linux-kernel - Re: warning: cast removes address space '_

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Date: Tue, 2 Apr 2024 09:56:24 +0200
From: Charlemagne Lasse <charlemagnelasse@...il.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: x86@...nel.org, LKML <linux-kernel@...r.kernel.org>, 
	Luc Van Oostenryck <lucvoo@...nel.org>, Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...nel.org>, 
	Nadav Amit <namit@...are.com>, Brian Gerst <brgerst@...il.com>, 
	Denys Vlasenko <dvlasenk@...hat.com>, "H . Peter Anvin" <hpa@...or.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: warning: cast removes address space '__percpu' of expression

Am Mo., 1. Apr. 2024 um 21:16 Uhr schrieb Uros Bizjak <ubizjak@...il.com>:
> > I would even go as far as saying that 1ca3683cc6d2 ("x86/percpu:
> > Enable named address spaces with known compiler version") together
> > with 3a1d3829e193 ("x86/percpu: Avoid sparse warning with cast to
> > named address space") triggered this problem

I think 1ca3683cc6d2 was wrong and is the last working one.


Just switch to 1ca3683cc6d2c2ce4204df519c4e4730d037905a and you won't
see the messages.

```
git reset --hard 1ca3683cc6d2c2ce4204df519c4e4730d037905a
git clean -dfx
make allnoconfig -j$(nproc)
make kvm_guest.config
make prepare -j$(nproc)
touch include/linux/netdevice.h
make C=1 net/core/dev.o CHECK="sparse -Wcast-from-as"
```

Go to 9a462b9eafa6dda16ea8429b151edb1fb535d744 and cherry-pick
3a1d3829e193c091475ceab481c5f8deab385023 and you would see the error.
On amd64  with 12.2.0, this would look like this:

```
git reset --hard 9a462b9eafa6dda16ea8429b151edb1fb535d744
git cherry-pick 3a1d3829e193c091475ceab481c5f8deab385023
git clean -dfx
make allnoconfig -j$(nproc)
make kvm_guest.config
make prepare -j$(nproc)
touch include/linux/netdevice.h
make C=1 net/core/dev.o CHECK="sparse -Wcast-from-as"
```

I would recommend to use `-Wsparse-all` for testing but for this
demonstration, it is easier to use `-Wcast-from-as` to reduce the
amount of noise in the demonstrator.