[PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE

[Pawan Gupta]

On CPUs that deprecated TSX, clearing the enumeration bits CPUID.RTM and
CPUID.HLE may not be desirable in some corner cases. Like a saved guest
would refuse to resume if it was saved before the microcode update
that deprecated TSX.

Add a cmdline option "tsx=fake" to not clear CPUID bits even when the
hardware always aborts TSX transactions.

Suggested-by: Tony Luck <tony.luck@xxxxxxxxx>
Suggested-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
Tested-by: Neelima Krishnan <neelima.krishnan@xxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt | 3 +++
 arch/x86/kernel/cpu/bugs.c                      | 5 +++--
 arch/x86/kernel/cpu/cpu.h                       | 1 +
 arch/x86/kernel/cpu/tsx.c                       | 7 +++++--
 4 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index cb89dbdedc46..ced9e5596163 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5693,6 +5693,9 @@
 			auto	- Disable TSX if X86_BUG_TAA is present,
 				  otherwise enable TSX on the system.
 
+			fake	- Do not clear the CPUID bits RTM and HLE even
+				  when hardware always aborts TSX transactions.
+
 			Not specifying this option is equivalent to tsx=off.
 
 			See Documentation/admin-guide/hw-vuln/tsx_async_abort.rst
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index d41b70fe4918..46fcc392a339 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -316,8 +316,9 @@ static void __init taa_select_mitigation(void)
 		return;
 	}
 
-	/* TSX previously disabled by tsx=off */
-	if (!boot_cpu_has(X86_FEATURE_RTM)) {
+	/* TSX previously disabled by tsx=off or by microcode */
+	if (!boot_cpu_has(X86_FEATURE_RTM) ||
+	     boot_cpu_has(X86_FEATURE_RTM_ALWAYS_ABORT)) {
 		taa_mitigation = TAA_MITIGATION_TSX_DISABLED;
 		goto out;
 	}
diff --git a/arch/x86/kernel/cpu/cpu.h b/arch/x86/kernel/cpu/cpu.h
index 95521302630d..84a479866c4b 100644
--- a/arch/x86/kernel/cpu/cpu.h
+++ b/arch/x86/kernel/cpu/cpu.h
@@ -49,6 +49,7 @@ enum tsx_ctrl_states {
 	TSX_CTRL_ENABLE,
 	TSX_CTRL_DISABLE,
 	TSX_CTRL_RTM_ALWAYS_ABORT,
+	TSX_CTRL_FAKE,
 	TSX_CTRL_NOT_SUPPORTED,
 };
 
diff --git a/arch/x86/kernel/cpu/tsx.c b/arch/x86/kernel/cpu/tsx.c
index 5ed99811504c..2f8e50584297 100644
--- a/arch/x86/kernel/cpu/tsx.c
+++ b/arch/x86/kernel/cpu/tsx.c
@@ -113,6 +113,8 @@ void __init tsx_init(void)
 			tsx_ctrl_state = TSX_CTRL_DISABLE;
 		} else if (!strcmp(arg, "auto")) {
 			tsx_ctrl_state = x86_get_tsx_auto_mode();
+		} else if (!strcmp(arg, "fake")) {
+			tsx_ctrl_state = TSX_CTRL_FAKE;
 		} else {
 			tsx_ctrl_state = TSX_CTRL_DISABLE;
 			pr_err("invalid option, defaulting to off\n");
@@ -131,9 +133,10 @@ void __init tsx_init(void)
 	 * Hardware will always abort a TSX transaction if both CPUID bits
 	 * RTM_ALWAYS_ABORT and TSX_FORCE_ABORT are enumerated.  In this case it
 	 * is better not to enumerate CPUID.RTM and CPUID.HLE bits. Clear them
-	 * here.
+	 * here, except when user requested not to clear via cmdline tsx=fake.
 	 */
-	if (boot_cpu_has(X86_FEATURE_RTM_ALWAYS_ABORT) &&
+	if (tsx_ctrl_state != TSX_CTRL_FAKE &&
+	    boot_cpu_has(X86_FEATURE_RTM_ALWAYS_ABORT) &&
 	    boot_cpu_has(X86_FEATURE_TSX_FORCE_ABORT)) {
 		tsx_ctrl_state = TSX_CTRL_RTM_ALWAYS_ABORT;
 		tsx_clear_cpuid();
-- 
git-series 0.9.1