linux-kernel - Re: [RFC][PATCH 2/7] x86, mpx: update MPX to grok larger bounds tables

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Date:   Sun, 12 Feb 2017 20:05:53 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        kirill.shutemov@...ux.intel.com
Subject: Re: [RFC][PATCH 2/7] x86, mpx: update MPX to grok larger bounds
 tables

On Wed, 1 Feb 2017, Dave Hansen wrote:
>  /*
> - * The upper 28 bits [47:20] of the virtual address in 64-bit
> - * are used to index into bounds directory (BD).
> + * The uppermost bits [56:20] of the virtual address in 64-bit
> + * are used to index into bounds directory (BD).  On processors
> + * with support for smaller virtual address space size, the "56"
> + * is obviously smaller.

 ... space size, the upper limit is adjusted accordingly.

Or something like that,

> +/*
> + * Note: size of tables on 64-bit is not constant, so we have no
> + * fixed definition for MPX_BD_NR_ENTRIES_64.
> + *
> + * The 5-Level Paging Whitepaper says:  "A bound directory
> + * comprises 2^(28+MAWA) 64-bit entries."  Since MAWA=0 in
> + * legacy mode:
> + */
> +#define MPX_BD_LEGACY_NR_ENTRIES_64	(1UL<<28)

(1UL << 28) please

>  
> +static inline int mpx_bd_size_shift(struct mm_struct *mm)
> +{
> +	return mm->context.mpx_bd_shift;
> +}

Do we really need that helper?

>  static inline unsigned long mpx_bd_size_bytes(struct mm_struct *mm)
>  {
> -	if (is_64bit_mm(mm))
> -		return MPX_BD_SIZE_BYTES_64;
> -	else
> +	if (!is_64bit_mm(mm))
>  		return MPX_BD_SIZE_BYTES_32;
> +
> +	/*
> +	 * The bounds directory grows with the address space size.
> +	 * The "legacy" shift is 0.
> +	 */
> +	return MPX_BD_BASE_SIZE_BYTES_64 << mpx_bd_shift_shift(mm);

shift_shift. I wonder how that compiles...

Looks good otherwise.

Thanks,

	tglx