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

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Message-ID: <20220715172113.74t65hgxi2o32wv6@desk>
Date:   Fri, 15 Jul 2022 10:21:13 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Greg KH <gregkh@...uxfoundation.org>
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,
        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 Fri, Jul 15, 2022 at 07:25:43AM +0200, Greg KH wrote:
> 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>

Sorry, I CCed stable@ by mistake. This version just adds a warning, it
is not intended to be backported.

Thanks,
Pawan