linux-kernel - Re: [PATCH v2] x86/bugs: Restructure ITS mitigation

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Message-ID: <20250520163221.ndezdfrhoav43sxi@desk>
Date: Tue, 20 May 2025 09:32:21 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: David Kaplan <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 v2] x86/bugs: Restructure ITS mitigation

On Fri, May 16, 2025 at 02:32:11PM -0500, David Kaplan wrote:
> Restructure the ITS mitigation to use select/update/apply functions like
> the other mitigations.
> 
> There is a particularly complex interaction between ITS and Retbleed as CDT
> (Call Depth Tracking) is a mitigation for both, and either its=stuff or
> retbleed=stuff will attempt to enable CDT.
> 
> retbleed_update_mitigation() runs first and will check the necessary
> pre-conditions for CDT if either ITS or Retbleed stuffing is selected.  If
> checks pass and ITS stuffing is selected, it will select stuffing for
> Retbleed as well.
> 
> its_update_mitigation() runs after and will either select stuffing if
> retbleed stuffing was enabled, or fall back to the default (aligned thunks)
> if stuffing could not be enabled.
> 
> Enablement of CDT is done exclusively in retbleed_apply_mitigation().
> its_apply_mitigation() is only used to enable aligned thunks.
> 
> Changes since v1:
>    - Moved ITS enum definition before retbleed logic
> 
> Signed-off-by: David Kaplan <david.kaplan@....com>
> ---
>  arch/x86/kernel/cpu/bugs.c | 167 ++++++++++++++++++++-----------------
...
> @@ -1338,20 +1365,6 @@ static void __init retbleed_apply_mitigation(void)
>  #undef pr_fmt
>  #define pr_fmt(fmt)     "ITS: " fmt
>  
> -enum its_mitigation_cmd {
> -	ITS_CMD_OFF,
> -	ITS_CMD_ON,
> -	ITS_CMD_VMEXIT,
> -	ITS_CMD_RSB_STUFF,
> -};
> -
> -enum its_mitigation {
> -	ITS_MITIGATION_OFF,
> -	ITS_MITIGATION_VMEXIT_ONLY,
> -	ITS_MITIGATION_ALIGNED_THUNKS,
> -	ITS_MITIGATION_RETPOLINE_STUFF,
> -};
> -
>  static const char * const its_strings[] = {
>  	[ITS_MITIGATION_OFF]			= "Vulnerable",

Index 1 (which is now ITS_MITIGATION_AUTO) is missing. I understand AUTO is
a temporary state, and it may not be necessary to define a string for it.
But, assigning an empty string, or an error message would make this obvious
for a future reader.

>  	[ITS_MITIGATION_VMEXIT_ONLY]		= "Mitigation: Vulnerable, KVM: Not affected",
> @@ -1359,11 +1372,6 @@ static const char * const its_strings[] = {
>  	[ITS_MITIGATION_RETPOLINE_STUFF]	= "Mitigation: Retpolines, Stuffing RSB",
>  };