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

[Pawan Gupta]

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[] = {