Re: [PATCH] x86/tsx: Use MSR_TSX_CTRL to clear CPUID bits

[Borislav Petkov]

On Mon, Feb 14, 2022 at 02:41:21PM -0800, Pawan Gupta wrote:
> Yes, this needs to be backported to a few kernels that have the commit
> 293649307ef9 ("x86/tsx: Clear CPUID bits when TSX always force aborts").
> Once this is reviewed, I will send a separate email to stable@ with the
> list of stable kernels.

You don't have to send a separate email - CC: stable and the Fixes tag
is enough for a patch to be picked up by the stable folks.

> X86_FEATURE_RTM_ALWAYS_ABORT is the precondition for
> MSR_TFA_TSX_CPUID_CLEAR bit to exist. For current callers of
> tsx_clear_cpuid() this condition is met, and test for
> X86_FEATURE_RTM_ALWAYS_ABORT can be removed. But, all the future callers
> must also have this check, otherwise the MSR write will fault.

I meant something like this (completely untested):

diff --git a/arch/x86/kernel/cpu/tsx.c b/arch/x86/kernel/cpu/tsx.c
index c2343ea911e8..9d08a6b1726a 100644
--- a/arch/x86/kernel/cpu/tsx.c
+++ b/arch/x86/kernel/cpu/tsx.c
@@ -84,7 +84,7 @@ static enum tsx_ctrl_states x86_get_tsx_auto_mode(void)
 	return TSX_CTRL_ENABLE;
 }
 
-void tsx_clear_cpuid(void)
+bool tsx_clear_cpuid(void)
 {
 	u64 msr;
 
@@ -97,11 +97,14 @@ void tsx_clear_cpuid(void)
 		rdmsrl(MSR_TSX_FORCE_ABORT, msr);
 		msr |= MSR_TFA_TSX_CPUID_CLEAR;
 		wrmsrl(MSR_TSX_FORCE_ABORT, msr);
+		return true;
 	} else if (tsx_ctrl_is_supported()) {
 		rdmsrl(MSR_IA32_TSX_CTRL, msr);
 		msr |= TSX_CTRL_CPUID_CLEAR;
 		wrmsrl(MSR_IA32_TSX_CTRL, msr);
+		return true;
 	}
+	return false;
 }
 
 void __init tsx_init(void)
@@ -114,9 +117,8 @@ void __init tsx_init(void)
 	 * RTM_ALWAYS_ABORT is set. In this case, it is better not to enumerate
 	 * CPUID.RTM and CPUID.HLE bits. Clear them here.
 	 */
-	if (boot_cpu_has(X86_FEATURE_RTM_ALWAYS_ABORT)) {
+	if (tsx_clear_cpuid()) {
 		tsx_ctrl_state = TSX_CTRL_RTM_ALWAYS_ABORT;
-		tsx_clear_cpuid();
 		setup_clear_cpu_cap(X86_FEATURE_RTM);
 		setup_clear_cpu_cap(X86_FEATURE_HLE);
 		return;

---

but I'm guessing TSX should be disabled by default during boot only when
X86_FEATURE_RTM_ALWAYS_ABORT is set.

If those CPUs which support only disabling TSX through MSR_IA32_TSX_CTRL
but don't have MSR_TSX_FORCE_ABORT - if those CPUs set
X86_FEATURE_RTM_ALWAYS_ABORT too, then this should work.

> There are certain cases where this will leave the system in an
> inconsistent state, for example smt toggle after a late microcode update

What is a "smt toggle"?

You mean late microcode update and then offlining and onlining all
logical CPUs except the BSP which would re-detect CPUID features?

> that adds CPUID.RTM_ALWAYS_ABORT=1. During an smt toggle, if we
> unconditionally clear CPUID.RTM and CPUID.HLE in init_intel(), half of
> the CPUs will report TSX feature and other half will not.

That is important and should be documented. Something like this perhaps:

---

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 8321c43554a1..6c7bca9d6f2e 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -722,6 +722,13 @@ static void init_intel(struct cpuinfo_x86 *c)
 	else if (tsx_ctrl_state == TSX_CTRL_DISABLE)
 		tsx_disable();
 	else if (tsx_ctrl_state == TSX_CTRL_RTM_ALWAYS_ABORT)
+		/*
+		 * This call doesn't clear RTM and HLE X86_FEATURE bits because
+		 * a late microcode reload adding MSR_TSX_FORCE_ABORT can cause
+		 * for those bits to get cleared - something which the kernel
+		 * cannot do due to userspace potentially already using said
+		 * features.
+		 */
 		tsx_clear_cpuid();
 
 	split_lock_init();

-- 
Regards/Gruss,
    Boris.

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