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

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Message-ID: <CAFGhKbzdWLw7fmRTLYPhEycWZvfjE-OCvnWct2A_kWVe94-p-A@mail.gmail.com>
Date: Tue, 2 Apr 2024 22:09:50 +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>, 
	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 Di., 2. Apr. 2024 um 11:53 Uhr schrieb Charlemagne Lasse
<charlemagnelasse@...il.com>:
>
> Am Di., 2. Apr. 2024 um 11:43 Uhr schrieb Uros Bizjak <ubizjak@...il.com>:
> [snip]
> > Please try the attached patch that informs sparse about this action.
>
> I've tested it using
>
> ```
> git reset --hard v6.9-rc2
> patch -p1 -i ~/p.diff.txt
> 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"
> ```
>
> And it seems to work. Thanks

But I found another problem which seem to have been introduced by
commit ed2f752e0e0a ("x86/percpu: Introduce const-qualified
const_pcpu_hot to micro-optimize code generation")


```
git reset --hard ed2f752e0e0a21d941ca0ee539ef3d4cd576bc5e
git cherry-pick 3a1d3829e193c091475ceab481c5f8deab385023
patch -p1 -i ~/p.diff.txt
git clean -dfx
make allnoconfig -j$(nproc)
make kvm_guest.config
echo CONFIG_MODULES=y >> .config
echo CONFIG_NET_9P_VIRTIO=m >> .config
make olddefconfig
make prepare -j$(nproc)
touch net/9p/trans_virtio.o
make C=1 M=net/9p/ trans_virtio.o CHECK="sparse -Wconstexpr-not-const"
```

This now shows the warning:

```
net/9p/trans_virtio.c:831:1: warning: non-constant initializer for static object
net/9p/trans_virtio.c:832:1: warning: non-constant initializer for static object
```

Which is from

```
module_init(p9_virtio_init);
module_exit(p9_virtio_cleanup);
```

The same happens when directly switching to the mentioned commit:

```
git reset --hard ed2f752e0e0a21d941ca0ee539ef3d4cd576bc5e
git cherry-pick 3a1d3829e193c091475ceab481c5f8deab385023
patch -p1 -i ~/p.diff.txt
git clean -dfx
make allnoconfig -j$(nproc)
make kvm_guest.config
echo CONFIG_MODULES=y >> .config
echo CONFIG_NET_9P_VIRTIO=m >> .config
make olddefconfig
make prepare -j$(nproc)
touch net/9p/trans_virtio.o
make C=1 M=net/9p/ trans_virtio.o CHECK="sparse -Wconstexpr-not-const"
```


So something for module_init and module_exit changed with this commit.
I can't see this when switching to a version before this commit:

```
git reset --hard ed2f752e0e0a21d941ca0ee539ef3d4cd576bc5e~1
git cherry-pick 3a1d3829e193c091475ceab481c5f8deab385023
git clean -dfx
make allnoconfig -j$(nproc)
make kvm_guest.config
echo CONFIG_MODULES=y >> .config
echo CONFIG_NET_9P_VIRTIO=m >> .config
make olddefconfig
make prepare -j$(nproc)
touch net/9p/trans_virtio.o
make C=1 M=net/9p/ trans_virtio.o CHECK="sparse -Wconstexpr-not-const"
```