[MODERATED] Re: ***UNCHECKED*** Re: Re: [PATCH v5 08/11] TAAv5 8

On Wed 16-10-19 09:26:14, speck for Jiri Kosina wrote:
> On Tue, 15 Oct 2019, speck for Michal Hocko wrote:
> 
> > From 9666e91b63cd6213d362d04289e1bcbbe2050bc3 Mon Sep 17 00:00:00 2001
> > From: Michal Hocko <[email protected]>
> > Date: Tue, 15 Oct 2019 11:21:01 +0200
> > Subject: [PATCH] x86, tsx: allow to set tsx=auto by a config option
> > 
> > There is a general consensus that TSX usage is not largely spread while
> > the history shows there is a non trivial space for side channel attacks
> > possible. Therefore the tsx is disabled by default even on platforms
> > that might have a safe implementation of TSX according to the current
> > knowledge. This is a fair trade off to make.
> > 
> > There are, however, workloads that really do benefit from using TSX and
> > updating to a newer kernel with TSX disabled might introduce a
> > noticeable regressions. This would be especially a problem for Linux
> > distributions which will provide TAA mitigations.
> > 
> > Introduce X86_INTEL_ENABLE_SAFE_TSX config option to override the
> > default tsx=off semantic and make tsx=auto a default which is more
> > update friendly.
> 
> So given the recent clarifications, especially around older MDS_NO=0 CPUs, 
> I think it'd actually make more sense to turn this into a config option 
> that would simply allow to chose what the default behavior should be, 
> chosing from tristate off/on/auto.

This on top of the commit?
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 9823e34b81ce..f7f79f189359 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1940,11 +1940,11 @@ config X86_INTEL_MEMORY_PROTECTION_KEYS
 
 	  If unsure, say y.
 
-config X86_INTEL_ENABLE_SAFE_TSX
-	prompt ""
-	def_bool n
+choice
+	prompt "TSX enable mode"
 	depends on CPU_SUP_INTEL
-	---help---
+	default X86_INTEL_TSX_MODE_OFF
+	help
 	  Intel's TSX (Transactional Synchronization Extensions) feature
 	  allows to optimize locking protocols through lock elision which
 	  can lead to a noticeable performance boost.
@@ -1953,14 +1953,37 @@ config X86_INTEL_ENABLE_SAFE_TSX
 	  to form side channel attacks (e.g. TAA) and chances are there
 	  will be more of those attacks discovered in the future.
 
-	  Therefore the TSX is not enabled by default. An admin might override
-	  this decision by tsx=on command line parameter. This has a risk that
-	  TSX will get enabled also on platforms which are known to be vulnerable
-	  to attacks like TAA and a safer option is to use tsx=auto command line
-	  parameter. Enabling this config option will make tsx=auto the default.
-	  See Documentation/admin-guide/kernel-parameters.txt for more details.
+	  Therefore the TSX is not enabled by default (aka tsx=off). An admin
+	  might override this decision by tsx=on command line parameter. This
+	  has a risk that TSX will get enabled also on platforms which are
+	  known to be vulnerable to attacks like TAA and a safer option is to
+	  use tsx=auto command line parameter.
 
-	  If you really benefit from TSX then enable this option, otherwise say n.
+	  This options allows to set the default tsx mode between tsx=on, off
+	  and auto. See Documentation/admin-guide/kernel-parameters.txt for more
+	  details.
+
+	  Say off if not sure, auto if TSX is in use but it should be used on safe
+	  platforms or on if TSX is in use and the security aspect of tsx in not
+	  relevant.
+
+config X86_INTEL_TSX_MODE_OFF
+	bool "off"
+	help
+	  TSX is always disabled - equals tsx=off command line parameter.
+
+config X86_INTEL_TSX_MODE_ON
+	bool "on"
+	help
+	  TSX is always enabled on TSX capable HW - equals tsx=on command line
+	  parameter.
+
+config X86_INTEL_TSX_MODE_AUTO
+	bool "auto"
+	help
+	  TSX is enabled on TSX capable HW that is believed to be safe against
+	  side channel attacks- equals tsx=auto command line parameter.
+endchoice
 
 config EFI
 	bool "EFI runtime service support"
diff --git a/arch/x86/kernel/cpu/tsx.c b/arch/x86/kernel/cpu/tsx.c
index d3dc1ce5cd4b..a2df0d5ffb15 100644
--- a/arch/x86/kernel/cpu/tsx.c
+++ b/arch/x86/kernel/cpu/tsx.c
@@ -99,10 +99,12 @@ void __init tsx_init(void)
 		}
 	} else {
 		/* tsx= not provided */
-		if (IS_ENABLED(X86_INTEL_ENABLE_SAFE_TSX))
+		if (IS_ENABLED(X86_INTEL_TSX_MODE_AUTO))
 			tsx_ctrl_state = x86_safe_tsx_mode();
-		else
+		else if (IS_ENABLED(X86_INTEL_TSX_MODE_OFF))
 			tsx_ctrl_state = TSX_CTRL_DISABLE;
+		else
+			tsx_ctrl_state = TSX_CTRL_ENABLE;
 	}
 
 	if (tsx_ctrl_state == TSX_CTRL_DISABLE) {
-- 
Michal Hocko
SUSE Labs