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Subject: Re: [PATCH] x86/percpu: Use __force to cast from __percpu address
 space
From: Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20240402175058.52649-1-ubizjak@gmail.com>
Date: Fri, 5 Apr 2024 17:38:13 +0300
Cc: the arch/x86 maintainers <x86@kernel.org>,
 Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
 Charlemagne Lasse <charlemagnelasse@gmail.com>,
 Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
 Ingo Molnar <mingo@kernel.org>,
 Andy Lutomirski <luto@kernel.org>,
 Brian Gerst <brgerst@gmail.com>,
 Denys Vlasenko <dvlasenk@redhat.com>,
 "H. Peter Anvin" <hpa@zytor.com>,
 Linus Torvalds <torvalds@linux-foundation.org>,
 Peter Zijlstra <peterz@infradead.org>,
 Thomas Gleixner <tglx@linutronix.de>,
 Josh Poimboeuf <jpoimboe@redhat.com>
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Message-Id: <0F9CDB7B-E8DD-43E5-9394-233A9289B3E0@gmail.com>
References: <20240402175058.52649-1-ubizjak@gmail.com>
To: Uros Bizjak <ubizjak@gmail.com>


> On 2 Apr 2024, at 20:50, Uros Bizjak <ubizjak@gmail.com> wrote:
>=20
> Fix sparse warning when casting from __percpu address space by using
> __force in the cast. x86 named address spaces are not considered to
> be subspaces of the generic (flat) address space, so explicit casts
> are required to convert pointers between these address spaces and the
> generic address space (the application should cast to uintptr_t and
> apply the segment base offset). The cast to uintptr_t removes
> __percpu address space tag and sparse reports:
>=20
>  warning: cast removes address space '__percpu' of expression
>=20
> Use __force to inform sparse that the cast is intentional.
>=20
> Reported-by: Charlemagne Lasse <charlemagnelasse@gmail.com>
> Closes: =
https://lore.kernel.org/lkml/CAFGhKbzev7W4aHwhFPWwMZQEHenVgZUj7=3DaunFieVq=
Zg3mt14A@mail.gmail.com/
> Fixes: 9a462b9eafa6 ("x86/percpu: Use compiler segment prefix =
qualifier")
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> Cc: Nadav Amit <nadav.amit@gmail.com>
> Cc: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Denys Vlasenko <dvlasenk@redhat.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> ---
> arch/x86/include/asm/percpu.h | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>=20
> diff --git a/arch/x86/include/asm/percpu.h =
b/arch/x86/include/asm/percpu.h
> index 1f6404e0c428..20696df5d567 100644
> --- a/arch/x86/include/asm/percpu.h
> +++ b/arch/x86/include/asm/percpu.h
> @@ -77,7 +77,7 @@
> #define arch_raw_cpu_ptr(_ptr) \
> ({ \
> unsigned long tcp_ptr__ =3D __raw_my_cpu_offset; \
> - tcp_ptr__ +=3D (unsigned long)(_ptr); \
> + tcp_ptr__ +=3D (__force unsigned long)(_ptr); \
> (typeof(*(_ptr)) __kernel __force *)tcp_ptr__; \
> })
> #else
> @@ -96,8 +96,8 @@
> #endif /* CONFIG_SMP */
>=20
> #define __my_cpu_type(var) typeof(var) __percpu_seg_override
> -#define __my_cpu_ptr(ptr) (__my_cpu_type(*ptr) *)(uintptr_t)(ptr)
> -#define __my_cpu_var(var) (*__my_cpu_ptr(&var))
> +#define __my_cpu_ptr(ptr) (__my_cpu_type(*ptr)*)(__force =
uintptr_t)(ptr)
> +#define __my_cpu_var(var) (*__my_cpu_ptr(&(var)))

If you use "(var)=E2=80=9D in __my_cpu_var(),, you might just as well =
change the first to:

  #define __my_cpu_ptr(ptr) (__my_cpu_type(*(ptr))*)(__force =
uintptr_t)(ptr)

[ not that I think of any real issue it might cause - just for =
consistency]

Anyhow,

Asked-by: Nadav Amit <nadav.amit@gmail.com>=