linux-kernel - Re: [PATCH -next for tip:x86/pti] x86/tlb: drop unneeded local vars in enable_l1d_flush_for

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Message-ID: <alpine.DEB.2.21.2009301848050.21555@felia>
Date:   Wed, 30 Sep 2020 18:53:37 +0200 (CEST)
From:   Lukas Bulwahn <lukas.bulwahn@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     Peter Zijlstra <peterz@...radead.org>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Balbir Singh <sblbir@...zon.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        kernel-janitors@...r.kernel.org, linux-safety@...ts.elisa.tech
Subject: Re: [PATCH -next for tip:x86/pti] x86/tlb: drop unneeded local vars
 in enable_l1d_flush_for_task()



On Wed, 30 Sep 2020, Thomas Gleixner wrote:

> On Tue, Sep 29 2020 at 10:37, Peter Zijlstra wrote:
> > Here, I fixed it..
> 
> Well, no. What Balbir is trying to do here is to establish whether a
> task runs on a !SMT core. sched_smt_active() is system wide, but their
> setup is to have a bunch of SMT enabled cores and cores where SMT is off
> because the sibling is offlined. They affine these processes to non SMT
> cores and the check there validates that before it enabled that flush
> thingy.
> 
> Of course this is best effort voodoo because if all CPUs in the mask are
> offlined then the task is moved to a SMT enabled one where L1D flush is
> useless. Though offlining their workhorse CPUs is probably not the daily
> business for obvious raisins.
>

Thanks, Thomas.

So, I will keep the semantics as-is, clean up the patch with 
preempt_{dis,en}able() and send out a v2.

Lukas