linux-kernel - RE: [PATCH v3 12/35] x86/bugs: Restructure retbleed mitigation

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Message-ID:
 <LV3PR12MB9265FF528F7AEB7E3373B2B394132@LV3PR12MB9265.namprd12.prod.outlook.com>
Date: Thu, 9 Jan 2025 16:42:43 +0000
From: "Kaplan, David" <David.Kaplan@....com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
CC: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3 12/35] x86/bugs: Restructure retbleed mitigation

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> Sent: Thursday, January 9, 2025 10:41 AM
> To: Kaplan, David <David.Kaplan@....com>
> Cc: Thomas Gleixner <tglx@...utronix.de>; Borislav Petkov <bp@...en8.de>; Peter
> Zijlstra <peterz@...radead.org>; Josh Poimboeuf <jpoimboe@...nel.org>; Ingo
> Molnar <mingo@...hat.com>; Dave Hansen <dave.hansen@...ux.intel.com>;
> x86@...nel.org; H . Peter Anvin <hpa@...or.com>; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH v3 12/35] x86/bugs: Restructure retbleed mitigation
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> On Thu, Jan 09, 2025 at 03:26:58PM +0000, Kaplan, David wrote:
> > The intent was never to allow AUTO to persist, it should always be
> > turned into a real mitigation.  However it looks like I did miss a
> > case there, where if the mitigation is AUTO when
> > retbleed_select_mitigation() is called, the bug should be mitigated
> > but the vendor isn't AMD/Hygon, it wasn't being transformed.
> >
> > I'll figure out how to fix this to match the existing functionality,
> > thanks for pointing this out.
>
> Also, adding a guard to ensure AUTO never persists would be good.
>
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index
> 5bc2782f4ce1..ad63b5678250 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1383,6 +1383,9 @@ static void __init retbleed_update_mitigation(void)
>                 }
>         }
>  out:
> +       if (retbleed_mitigation == RETBLEED_MITIGATION_AUTO)
> +               retbleed_mitigation = RETBLEED_MITIGATION_NONE;
> +
>         pr_info("%s\n", retbleed_strings[retbleed_mitigation]);
>  }

I had the same idea, I think to make this work I will let the mitigation stay as AUTO until the end of the update function where it will then be turned into NONE, at least for Intel.

For AMD, I can ensure it always is transformed in the select mitigation function.

Thanks --David Kaplan