linux-kernel - Re: [PATCH v5 08/13] x86/fsgsbase/64: Introduce the FIND_PERCPU

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Date:   Sat, 2 Feb 2019 09:17:22 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Markus T Metzger <markus.t.metzger@...el.com>,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v5 08/13] x86/fsgsbase/64: Introduce the FIND_PERCPU_BASE macro

On Fri, Feb 1, 2019 at 12:55 PM Chang S. Bae <chang.seok.bae@...el.com> wrote:
>
> GSBASE is used to find per-CPU data in the kernel. But when it is unknown,
> the per-CPU base can be found from the per_cpu_offset table with a CPU NR.
> The CPU NR is extracted from the limit field of the CPUNODE entry in GDT,
> or by the RDPID instruction.
>
> Also, add the GAS-compatible RDPID macro.
>
> The new macro will be used on a following patch.
>
> Suggested-by: H. Peter Anvin <hpa@...or.com>
> Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Andi Kleen <ak@...ux.intel.com>
> ---
>  arch/x86/include/asm/fsgsbase.h | 46 +++++++++++++++++++++++++++++++++
>  arch/x86/include/asm/inst.h     | 15 +++++++++++
>  2 files changed, 61 insertions(+)
>
> diff --git a/arch/x86/include/asm/fsgsbase.h b/arch/x86/include/asm/fsgsbase.h
> index aefd53767a5d..eecca2250748 100644
> --- a/arch/x86/include/asm/fsgsbase.h
> +++ b/arch/x86/include/asm/fsgsbase.h
> @@ -78,6 +78,52 @@ extern void x86_gsbase_write_cpu_inactive(unsigned long gsbase);
>
>  #endif /* CONFIG_X86_64 */
>
> +#else /* __ASSEMBLY__ */
> +
> +#ifdef CONFIG_X86_64
> +
> +#include <asm/inst.h>
> +
> +#if CONFIG_SMP

ifdef?

> +
> +/*
> + * CPU/node NR is loaded from the limit (size) field of a special segment
> + * descriptor entry in GDT.
> + */
> +.macro LOAD_CPU_AND_NODE_SEG_LIMIT reg:req
> +       movq    $__CPUNODE_SEG, \reg
> +       lsl     \reg, \reg
> +.endm
> +

Please put the alternative in here instead of in FIND_PERCPU_BASE.

> +/*
> + * Fetch the per-CPU GSBASE value for this processor and put it in @reg.
> + * We normally use %gs for accessing per-CPU data, but we are setting up
> + * %gs here and obviously can not use %gs itself to access per-CPU data.
> + */
> +.macro FIND_PERCPU_BASE reg:req
> +       /*
> +        * The CPU/node NR is initialized earlier, directly in cpu_init().
> +        * The CPU NR is extracted from it.
> +        */

This comment is unnecessary.

> +       ALTERNATIVE \
> +               "LOAD_CPU_AND_NODE_SEG_LIMIT \reg", \
> +               "RDPID  \reg", \
> +               X86_FEATURE_RDPID
> +       andq    $VDSO_CPUNODE_MASK, \reg
> +       movq    __per_cpu_offset(, \reg, 8), \reg
> +.endm
> +
> +#else
> +
> +.macro FIND_PERCPU_BASE reg:req
> +       /* Tracking the base offset value */

I don't understand this comment at all.  Please just remove it.