linux-kernel - Re: [PATCH 06/10] mm,x86: skip cr4 and ldt reload when mm stays the same

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Date:   Sat, 28 Jul 2018 21:21:41 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
        Dave Hansen <dave.hansen@...el.com>, will.daecon@....com,
        Catalin Marinas <catalin.marinas@....com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 06/10] mm,x86: skip cr4 and ldt reload when mm stays the same

On Sat, Jul 28, 2018 at 2:53 PM, Rik van Riel <riel@...riel.com> wrote:
> When switching back from lazy TLB mode to a thread of the same process
> that switched into lazy TLB mode, we still have the cr4 (and sometimes
> LDT) of that process loaded, and there is no need to reload it.
>
> When there was no TLB flush while the CPU was in lazy TLB mode, the
> current code in switch_mm_irqs_off already avoids the reload, by
> returning early.
>
> However, when the TLB contents on the CPU are out of date, and we
> flush the TLB for the task, we fall through to the regular context
> switching code. This patch teaches that code to skip the cr4 and LDT
> flushes when switching back to the same mm after a flush.

Acked-by: Andy Lutomirski <luto@...nel.org>