linux-kernel - Re: [PATCH] x86: Align TLB invalidation info

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Date:   Wed, 31 Jan 2018 13:00:35 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH] x86: Align TLB invalidation info

On Wed, Jan 31, 2018 at 12:48 PM, Nadav Amit <nadav.amit@...il.com> wrote:
> Andy Lutomirski <luto@...capital.net> wrote:
>
>>
>>
>>> On Jan 31, 2018, at 12:11 PM, Nadav Amit <namit@...are.com> wrote:
>>>
>>> The TLB invalidation info is allocated on the stack, which might cause
>>> it to be unaligned. Since this information may be transferred to
>>> different cores for TLB shootdown, this might result in an additional
>>> cache-line bouncing between the cores.
>>>
>>> GCC provides a way to deal with it by using
>>> __builtin_alloca_with_align(). Use it to avoid the bouncing cache lines.
>>
>> Eww.  How about __aligned?
>
> Err.. Stupid me. For some reason I remembered I tried it and it didn’t have
> the desired effect, which caused me to assume it does not work for variables
> on the stack. Anyhow, it does the work. I’ll submit v2.
>

You're probably remembering that __aligned(16) malfunctions on older
GCC versions.  But __aligned(64), which is what you want, has always
been okay AFAIK.