linux-kernel - Re: [PATCH 1/3] x86/bugs: Create an option to disable MDS

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Date:   Thu, 15 Jun 2023 15:13:47 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Breno Leitao <leitao@...ian.org>
Cc:     jpoimboe@...nel.org, peterz@...radead.org, tglx@...utronix.de,
        bp@...en8.de, Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, leit@...com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] x86/bugs: Create an option to disable MDS

On Thu, Jun 15, 2023 at 09:44:12AM -0700, Breno Leitao wrote:
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 182af64387d0..50f12829dce9 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -269,6 +269,7 @@ static void x86_amd_ssb_disable(void)
>  /* Default mitigation for MDS-affected CPUs */
>  static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL;
>  static bool mds_nosmt __ro_after_init = false;
> +#define MDS_WARN_MSG "WARNING: Microarchitectural Data Sampling (MDS) speculative mitigation disabled!\n"
>  
>  static const char * const mds_strings[] = {
>  	[MDS_MITIGATION_OFF]	= "Vulnerable",
> @@ -278,11 +279,17 @@ static const char * const mds_strings[] = {
>  
>  static void __init mds_select_mitigation(void)
>  {
> -	if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off()) {
> +	if (!boot_cpu_has_bug(X86_BUG_MDS)) {
>  		mds_mitigation = MDS_MITIGATION_OFF;
>  		return;
>  	}
>  
> +	if (cpu_mitigations_off() || !IS_ENABLED(CONFIG_MITIGATE_MDS)) {
> +		mds_mitigation = MDS_MITIGATION_OFF;
> +		pr_err(MDS_WARN_MSG);
> +		return;

Why does compile time config needs to be so restrictive that it does not
allow runtime override with mds= cmdline?

I believe Kconfig should only be setting the mitigation default,
allowing users to select mitigation at runtime:

---
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 182af64387d0..50e1ca4ea68b 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -267,7 +267,11 @@ static void x86_amd_ssb_disable(void)
 #define pr_fmt(fmt)	"MDS: " fmt
 
 /* Default mitigation for MDS-affected CPUs */
+#if IS_ENABLED(CONFIG_MITIGATE_MDS)
 static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL;
+#else
+static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_OFF;
+#endif
 static bool mds_nosmt __ro_after_init = false;
 
 static const char * const mds_strings[] = {