linux-kernel - Re: [PATCH 2/2] x86/speculation: switch_to_cond

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Message-ID: <3732de84-a14f-e32a-61de-29c5965dd405@redhat.com>
Date:   Thu, 6 Dec 2018 10:38:00 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 2/2] x86/speculation: switch_to_cond_stibp on is the
 likely case

On 12/06/2018 03:41 AM, Peter Zijlstra wrote:
> On Wed, Dec 05, 2018 at 02:49:28PM -0500, Waiman Long wrote:
>> Since conditional STIBP is the default, it should be treated as
>> the likely case. Changes the use of static_branch_unlikely() to
>> static_branch_likely() for switch_to_cond_stibp.
> So now you're making kernels on 'fixed' or unaffected hardware slower.

Good point.

The reason I sent out this patch is because of the inconsistency in the
use of likely/unlikely hints.

arch/x86/kernel/cpu/bugs.c:156:        if
(static_branch_unlikely(&switch_to_cond_stibp))
arch/x86/kernel/process.c:440:       
static_branch_unlikely(&switch_to_cond_stibp)) {
arch/x86/kernel/process.h:26:        if
(!static_branch_likely(&switch_to_cond_stibp)) {

So if we are aiming to optimize for "fixed" or unaffected hardware,
maybe we should modify the likely hint to unlikely then.

Cheers,
Longman