linux-kernel - Re: [PATCH v3 18/26] arm64: Introduce asm/vdso/processor.h

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Date:   Mon, 16 Mar 2020 10:22:15 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org,
        clang-built-linux@...glegroups.com, x86@...nel.org,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        Paul Burton <paul.burton@...s.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Collingbourne <pcc@...gle.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrei Vagin <avagin@...nvz.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v3 18/26] arm64: Introduce asm/vdso/processor.h

Hi Vincenzo,

On Mon, Mar 16, 2020 at 09:42:32AM +0000, Vincenzo Frascino wrote:
> On 3/15/20 6:30 PM, Catalin Marinas wrote:
> > On Fri, Mar 13, 2020 at 03:43:37PM +0000, Vincenzo Frascino wrote:
> >> --- /dev/null
> >> +++ b/arch/arm64/include/asm/vdso/processor.h
> >> @@ -0,0 +1,31 @@
> >> +/* SPDX-License-Identifier: GPL-2.0-only */
> >> +/*
> >> + * Copyright (C) 2020 ARM Ltd.
> >> + */
> >> +#ifndef __ASM_VDSO_PROCESSOR_H
> >> +#define __ASM_VDSO_PROCESSOR_H
> >> +
> >> +#ifndef __ASSEMBLY__
> >> +
> >> +#include <asm/page-def.h>
> >> +
> >> +#ifdef CONFIG_COMPAT
> >> +#if defined(CONFIG_ARM64_64K_PAGES) && defined(CONFIG_KUSER_HELPERS)
> >> +/*
> >> + * With CONFIG_ARM64_64K_PAGES enabled, the last page is occupied
> >> + * by the compat vectors page.
> >> + */
> >> +#define TASK_SIZE_32		UL(0x100000000)
> >> +#else
> >> +#define TASK_SIZE_32		(UL(0x100000000) - PAGE_SIZE)
> >> +#endif /* CONFIG_ARM64_64K_PAGES */
> >> +#endif /* CONFIG_COMPAT */
> > 
> > Just curious, what's TASK_SIZE_32 used for in the vDSO code? You don't
> > seem to move TASK_SIZE.
> > 
> 
> I tried to fine grain the headers as much as I could in order to avoid
> unneeded/unwanted inclusions:
>  * TASK_SIZE_32 is used to verify ABI consistency on vdso32 (please refer to
>    arch/arm64/kernel/vdso32/vgettimeofday.c).
>  * TASK_SIZE is not required by the vdso library hence it is not present in
>    these headers.

It would be worth noting the former point in the commit message, since
it can be surprising.

I also think it's worth keeping the definitions together if that's easy,
as it makes it easier to navigate the codebase, even if TASK_SIZE isn't
necessary for the VDSO itself.

Thanks,
Mark.