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Date:   Thu, 15 Jun 2023 15:13:47 -0700
From:   Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
To:     Breno Leitao <leitao@debian.org>
Cc:     jpoimboe@kernel.org, peterz@infradead.org, tglx@linutronix.de,
        bp@alien8.de, Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, leit@fb.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] x86/bugs: Create an option to disable MDS
Message-ID: <20230615221347.6ghb4d33tituxht4@desk>
References: <20230615164417.3588162-1-leitao@debian.org>
 <20230615164417.3588162-2-leitao@debian.org>
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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[] = {