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

[Pawan Gupta]

On 15.02.2022 00:28, Borislav Petkov wrote:
> On Mon, Feb 14, 2022 at 02:41:21PM -0800, Pawan Gupta wrote:
> > 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);

This will clear TSX CPUID when X86_FEATURE_RTM_ALWAYS_ABORT=0 also, because ...

> +		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()) {

... we are calling tsx_clear_cpuid() unconditionally.

> 		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.

That is correct.

> 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"?

Sorry, I should have been more clear. I meant:

	# echo off > /sys/devices/system/cpu/smt/control
	# echo on  > /sys/devices/system/cpu/smt/control

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

That could also be the problematic case.

> > 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();

Thanks, I will add this comment in the next revision.

Pawan