linux-kernel - Re: [PATCH 2/3] x86/cpu/amd: Add feature bit for MSR_AMD64_LS

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Message-ID: <20221109214126.ao477sucsks6msyd@desk>
Date:   Wed, 9 Nov 2022 13:41:26 -0800
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Pavel Machek <pavel@....cz>,
        "hdegoede@...hat.com" <hdegoede@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        "antonio.gomez.iglesias@...ux.intel.com" 
        <antonio.gomez.iglesias@...ux.intel.com>
Subject: Re: [PATCH 2/3] x86/cpu/amd: Add feature bit for MSR_AMD64_LS_CFG
 enumeration

On Wed, Nov 09, 2022 at 07:34:55PM +0100, Borislav Petkov wrote:
>On Wed, Nov 09, 2022 at 09:37:20AM -0800, Pawan Gupta wrote:
>> Looks like we need to restore this MSR too,
>
>Why do we? Is that MSR read-only too or what is the reason for that one?

Please ignore, it doesn't concern this series.

>Yes, MSR_AMD64_LS_CFG is used in SSBD mitigations. For everything <= 0x12:
>
>        /* AMD Family 0xf - 0x12 */
>        VULNWL_AMD(0x0f,        NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO),
>        VULNWL_AMD(0x10,        NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO),
>        VULNWL_AMD(0x11,        NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO),
>        VULNWL_AMD(0x12,        NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO),
>
>On F14h it says here:
>
>[    0.278930] Speculative Store Bypass: Vulnerable
>
>simply because it is not present there.

So looks like MSR_AMD64_LS_CFG should only be restored when
X86_FEATURE_LS_CFG_SSBD is present. I will make this change in v2.