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

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Message-ID: <20220715020005.dixdi3qpb23iz6up@desk>
Date:   Thu, 14 Jul 2022 19:00:05 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Thadeu Lima de Souza Cascardo <cascardo@...onical.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 09:11:18PM -0300, Thadeu Lima de Souza Cascardo 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>
>
>Reviewed-by: Thadeu Lima de Souza Cascardo <cascardo@...onical.com>

Thanks for the review.

Pawan