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Message-ID: <3F9D776E-AD7E-4814-9E3C-508550AD9287@vmware.com>
Date: Wed, 18 Oct 2023 14:46:23 +0000
From: Nadav Amit <namit@...are.com>
To: Uros Bizjak <ubizjak@...il.com>
CC: Linus Torvalds <torvalds@...ux-foundation.org>,
the arch/x86 maintainers <x86@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Andy Lutomirski <luto@...nel.org>,
Brian Gerst <brgerst@...il.com>,
Denys Vlasenko <dvlasenk@...hat.com>,
"H . Peter Anvin" <hpa@...or.com>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()
> On Oct 18, 2023, at 4:15 PM, Uros Bizjak <ubizjak@...il.com> wrote:
>
>>
>> Looks like another case of underspecified functionality where both
>> compilers differ. Luckily, both DTRT when aliases are hidden in
>> another TU.
>
> Attached is the prototype patch that works for me (together with
> Linus' FPU switching patch).
In general looks good. See some minor issues below.
> --- a/arch/x86/include/asm/current.h
> +++ b/arch/x86/include/asm/current.h
> @@ -36,10 +36,23 @@ static_assert(sizeof(struct pcpu_hot) == 64);
>
> DECLARE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot);
>
> +/*
> + *
> + */
Obviously some further comments to clarify why struct pcpu_hot is
defined in percpu-hot.c (the GCC manual says: "It is an error if
the alias target is not defined in the same translation unit as the
alias” which can be used as part of the explanation.)
> +DECLARE_PER_CPU_ALIGNED(const struct pcpu_hot __percpu_seg_override,
> + const_pcpu_hot);
> +
> +#ifdef CONFIG_USE_X86_SEG_SUPPORT
> +static __always_inline struct task_struct *get_current(void)
> +{
> + return const_pcpu_hot.current_task;
> +}
> +#else
> static __always_inline struct task_struct *get_current(void)
> {
> return this_cpu_read_stable(pcpu_hot.current_task);
> }
> +#endif
Please consider using IS_ENABLED() to avoid the ifdef’ry.
So this would turn to be:
static __always_inline struct task_struct *get_current(void)
{
if (IS_ENABLED(CONFIG_USE_X86_SEG_SUPPORT))
return const_pcpu_hot.current_task;
return this_cpu_read_stable(pcpu_hot.current_task);
}
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