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Message-ID: <20230615221347.6ghb4d33tituxht4@desk>
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[] = {
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