linux-kernel - Re: [PATCH V2 05/15] x86/vdso: Move out the CPU number store

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Date:   Thu, 31 May 2018 13:25:12 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     Andrew Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Metzger, Markus T" <markus.t.metzger@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 05/15] x86/vdso: Move out the CPU number store

On Thu, May 31, 2018 at 10:58 AM Chang S. Bae <chang.seok.bae@...el.com> wrote:
>
> The CPU (and node) number will be written, as early enough,
> to the segment limit of per CPU data and TSC_AUX MSR entry.
> The information has been retrieved by vgetcpu in user space
> and will be also loaded from the paranoid entry, when
> FSGSBASE enabled. So, it is moved out from vDSO to the CPU
> initialization path where IST setup is serialized.
>
> Now, redundant setting of the segment in entry/vdso/vma.c
> was removed; a substantial code removal. It removes a
> hotplug notifier, makes a facility useful to both the kernel
> and userspace unconditionally available much sooner, and
> unification with i386. (Thanks to HPA for suggesting the
> cleanup)
>
> Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Andi Kleen <ak@...ux.intel.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>

> +/* Bit size and mask of CPU number stored in the per CPU data */
> +#define PERCPU_CPU_SIZE                        12
> +#define PERCPU_CPU_MASK                        0xfff

This name is confusing.  Maybe LSL_TSCP_CPU_MASK?

Can you also add a helpers like:

static inline unsigned long make_lsl_tscp(unsigned int cpu, unsigned
int node) { ... }
static inline unsigned int lsl_tscp_to_cpu(unsigned long x) { ... }
static inline unsigned int lsl_tscp_to_node(unsigned long x) { ... }

and use them everywhere?  This will make it lot easier to understand the code.

> +#ifdef CONFIG_X86_32
> +                       GDT_ENTRY_PERCPU,
> +#else /* 64 bit */
> +                       GDT_ENTRY_PER_CPU,
>  #endif

Please just rename one of these to match the other one.