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Date: Sun, 12 Feb 2017 20:37:57 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Dave Hansen <dave.hansen@linux.intel.com>
cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
        kirill.shutemov@linux.intel.com
Subject: Re: [RFC][PATCH 4/7] x86, mpx: context-switch new MPX address size
 MSR
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On Wed, 1 Feb 2017, Dave Hansen wrote:
> +/*
> + * The MPX tables change sizes based on the size of the virtual
> + * (aka. linear) address space.  There is an MSR to tell the CPU
> + * whether we want the legacy-style ones or the larger ones when
> + * we are running with an eXtended virtual address space.
> + */
> +static inline void switch_mpx_bd(struct mm_struct *prev, struct mm_struct *next)
> +{
> +	/*
> +	 * Note: there is one and only one bit in use in the MSR
> +	 * at this time, so we do not have to be concerned with
> +	 * preserving any of the other bits.  Just write 0 or 1.
> +	 */
> +	u32 IA32_MPX_LAX_ENABLE_MASK = 0x00000001;
> +
> +	/*
> +	 * Avoid the MSR on CPUs without MPX, obviously:
> +	 */
> +	if (!cpu_feature_enabled(X86_FEATURE_MPX))
> +		return;
> +	/*
> +	 * FIXME: do we want a check here for the 5-level paging
> +	 * CR4 bit or CPUID bit, or is the mawa check below OK?
> +	 * It's not obvious what would be the fastest or if it
> +	 * matters.
> +	 */

Well, you could use a static key which is enabled when 5 level paging and
MPX is enabled.

> +	/*
> +	 * Avoid the relatively costly MSR if we are not changing
> +	 * MAWA state.  All processes not using MPX will have a
> +	 * mpx_mawa_shift()=0, so we do not need to check
> +	 * separately for whether MPX management is enabled.
> +	 */
> +	if (likely(mpx_bd_size_shift(prev) == mpx_bd_size_shift(next)))
> +		return;

So this switches back unconditionally if the previous task was using the
large tables even if the next task is not using MPX at all. It's probably a
non issue.

Thanks,

	tglx