linux-kernel - Re: [PATCH 3/9] x86: vdso: Introduce asm/vdso/page.h

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Message-ID: <a298ba4e-cbbf-4f50-b175-8ee3063963bc@csgroup.eu>
Date: Wed, 4 Sep 2024 17:05:19 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Arnd Bergmann <arnd@...db.de>,
 Vincenzo Frascino <vincenzo.frascino@....com>, linux-kernel@...r.kernel.org,
 Linux-Arch <linux-arch@...r.kernel.org>, linux-mm@...ck.org
Cc: Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 "Jason A . Donenfeld" <Jason@...c4.com>,
 Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>,
 Naveen N Rao <naveen@...nel.org>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, Theodore Ts'o <tytso@....edu>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH 3/9] x86: vdso: Introduce asm/vdso/page.h

Le 04/09/2024 à 16:52, Arnd Bergmann a écrit :
> On Tue, Sep 3, 2024, at 15:14, Vincenzo Frascino wrote:
> 
>> diff --git a/arch/x86/include/asm/vdso/page.h b/arch/x86/include/asm/vdso/page.h
>> new file mode 100644
>> index 000000000000..b0af8fbef27c
>> --- /dev/null
>> +++ b/arch/x86/include/asm/vdso/page.h
>> @@ -0,0 +1,15 @@
>> +
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +#ifndef __ASM_VDSO_PAGE_H
>> +#define __ASM_VDSO_PAGE_H
>> +
>> +#ifndef __ASSEMBLY__
>> +
>> +#include <asm/page_types.h>
>> +
>> +#define VDSO_PAGE_MASK	PAGE_MASK
>> +#define VDSO_PAGE_SIZE	PAGE_SIZE
>> +
>> +#endif /* !__ASSEMBLY__ */
>> +
>> +#endif /* __ASM_VDSO_PAGE_H */
> 
> I don't get this one: the x86 asm/page_types.h still includes other
> headers outside of the vdso namespace, but you seem to only need these
> two definitions that are the same across everything.
> 
> Why not put PAGE_MASK and PAGE_SIZE into a global vdso/page.h
> header? I did spend a lot of time a few months ago ensuring that
> we can have a single definition for all architectures based on
> CONFIG_PAGE_SHIFT, so all the extra copies should just go away.
> 

Just wondering, after looking at x86, powerpc and arm64, is there any 
difference between:

X86,ARM64:
#define PAGE_SIZE		(_AC(1,UL) << PAGE_SHIFT)
#define PAGE_MASK		(~(PAGE_SIZE-1))

POWERPC:
#define PAGE_SIZE		(ASM_CONST(1) << PAGE_SHIFT)
/*
  * Subtle: (1 << PAGE_SHIFT) is an int, not an unsigned long. So if we
  * assign PAGE_MASK to a larger type it gets extended the way we want
  * (i.e. with 1s in the high bits)
  */
#define PAGE_MASK      (~((1 << PAGE_SHIFT) - 1))

Which one should be taken in vdso/page.h ?

Christophe