linux-kernel - Re: [PATCH v3 02/11] x86/ldt: Simplify LDT switching logic

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Date:   Wed, 21 Jun 2017 11:40:15 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 02/11] x86/ldt: Simplify LDT switching logic

On Tue, Jun 20, 2017 at 10:22:08PM -0700, Andy Lutomirski wrote:
> Originally, Linux reloaded the LDT whenever the prev mm or the next
> mm had an LDT.  It was changed in 0bbed3beb4f2 ("[PATCH]
> Thread-Local Storage (TLS) support") (from the historical tree) like
> this:
> 
> -		/* load_LDT, if either the previous or next thread
> -		 * has a non-default LDT.
> +		/*
> +		 * load the LDT, if the LDT is different:
> 		 */
> -		if (next->context.size+prev->context.size)
> +		if (unlikely(prev->context.ldt != next->context.ldt))
> 			load_LDT(&next->context);
> 
> The current code is unlikely to avoid any LDT reloads, since different
> mms won't share an LDT.
> 
> When we redo lazy mode to stop flush IPIs without switching to
> init_mm, though, the current logic would become incorrect: it will
> be possible to have real_prev == next but nonetheless have a stale
> LDT descriptor.
> 
> Simplify the code to update LDTR if either the previous or the next
> mm has an LDT, i.e. effectively restore the historical logic..
> While we're at it, clean up the code by moving all the ifdeffery to
> a header where it belongs.
> 
> Acked-by: Rik van Riel <riel@...hat.com>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/include/asm/mmu_context.h | 26 ++++++++++++++++++++++++++
>  arch/x86/mm/tlb.c                  | 20 ++------------------
>  2 files changed, 28 insertions(+), 18 deletions(-)

Reviewed-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

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