linux-kernel - Re: [PATCH v2 2/4] x86/split_lock: Align x86_capability to unsigned long to avoid split locked access

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Date:   Sat, 30 Jun 2018 08:23:59 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
cc:     Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v2 2/4] x86/split_lock: Align x86_capability to unsigned
 long to avoid split locked access

On Fri, 29 Jun 2018, Fenghua Yu wrote:
> On Fri, Jun 29, 2018 at 05:00:51PM -0700, Fenghua Yu wrote:

> diff --git a/arch/x86/boot/cpuflags.h b/arch/x86/boot/cpuflags.h
> index 2e20814d3ce3..29de0ff74351 100644
> --- a/arch/x86/boot/cpuflags.h
> +++ b/arch/x86/boot/cpuflags.h
> @@ -9,7 +9,7 @@ struct cpu_features {
>  	int level;		/* Family, or 64 for x86-64 */
>  	int family;		/* Family, always */
>  	int model;
> -	u32 flags[NCAPINTS];
> +	u32 flags[NCAPINTS] __aligned(sizeof(unsigned long));

Lacks a comment WHY this needs the aligned() as Dave already said.

>  };
>  
>  extern struct cpu_features cpu;
> diff --git a/arch/x86/include/asm/mce.h b/arch/x86/include/asm/mce.h
> index 8c7b3e5a2d01..444a2275c1f8 100644
> --- a/arch/x86/include/asm/mce.h
> +++ b/arch/x86/include/asm/mce.h
> @@ -133,7 +133,7 @@ struct mce_log_buffer {
>  	char signature[12]; /* "MACHINECHECK" */
>  	unsigned len;	    /* = MCE_LOG_LEN */
>  	unsigned next;
> -	unsigned flags;
> +	unsigned flags __aligned(sizeof(unsigned long));

And whats wrong with just making that flags field unsigned long and move it
behind recordlen?

flags is at offset 20 so forcing alignement puts it at offset 24 creating a
4 byte hole. While reordering it and making it type unsigned long just
fills the already existing hole and just works w/o the stray aligned()

struct mce_log_buffer {
        char                       signature[12];        /*     0    12 */
        unsigned int               len;                  /*    12     4 */
        unsigned int               next;                 /*    16     4 */
        unsigned int               flags;                /*    20     4 */
        unsigned int               recordlen;            /*    24     4 */

        /* XXX 4 bytes hole, try to pack */

        struct mce                 entry[32];            /*    32  3840 */

The point is, that there is no requirement that flags is unsigned int,
while the u32 type for the capability array has a reason.

So you need to analyse first whether the data type is required or can be
changed to unsigned long which is the right thing to do.

See?

Thanks,

	tglx