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

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Date:   Tue, 8 Nov 2022 18:18:29 -0800
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Pavel Machek <pavel@....cz>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>, hdegoede@...hat.com,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Daniel Sneddon <daniel.sneddon@...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 12:10:32AM +0100, Borislav Petkov wrote:
>On Tue, Nov 08, 2022 at 02:51:41PM -0800, Pawan Gupta wrote:
>> Looking at bsp_init_amd() this feature bit will only be set on AMD
>> families 0x15-0x17. Andrew mentioned that the MSR LS_CFG is present on
>> AMD family >= 0x10 && family <= 0x18.
>
>Do you need to save that MSR on those families?
>
>Or do 0x15-0x18 suffice?
>
>Yes, 0x18 because that's Hygon and that does its own detection.
>
>So, do you need to save it on families 0x10-0x14?

As per Andrew's comment [1] the MSR needs to be restored for dispatch
serialising bit(except for family 0x11) :

   AMD LS_CFG is more complicated, because the dispatch serialising bit
   needs setting unilaterally (families 0x10, 0x12 thru 0x18), but the SSBD
   control ought to resolve on the next context switch.

[1] https://lore.kernel.org/lkml/6049e5bc-122f-5b4c-c1dc-0591eccf3525@citrix.com/