linux-kernel - Re: [PATCH v2] x86/bugs: Warn when "ibrs" mitigation is selected on Enhanced IBRS parts

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Date:   Fri, 15 Jul 2022 07:25:43 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        antonio.gomez.iglesias@...ux.intel.com,
        Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [PATCH v2] x86/bugs: Warn when "ibrs" mitigation is selected on
 Enhanced IBRS parts

On Thu, Jul 14, 2022 at 04:15:35PM -0700, Pawan Gupta wrote:
> IBRS mitigation for spectre_v2 forces write to MSR_IA32_SPEC_CTRL at
> every kernel entry/exit. On Enhanced IBRS parts setting
> MSR_IA32_SPEC_CTRL[IBRS] only once at boot is sufficient. MSR writes at
> every kernel entry/exit incur unnecessary performance loss.
> 
> When Enhanced IBRS feature is present, print a warning about this
> unnecessary performance loss.
> 
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> ---
> v1->v2: Instead of changing the mitigation, print a warning about the
>         perf loss.
> 
> v1: https://lore.kernel.org/lkml/0456b35fb9ef957d9a9138e0913fb1a3fd445dff.1657747493.git.pawan.kumar.gupta@linux.intel.com/
> 
>  arch/x86/kernel/cpu/bugs.c | 3 +++
>  1 file changed, 3 insertions(+)

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>