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Date:   Mon, 14 Feb 2022 14:41:21 -0800
From:   Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
To:     Borislav Petkov <bp@alien8.de>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, Andi Kleen <ak@linux.intel.com>,
        Tony Luck <tony.luck@intel.com>, linux-kernel@vger.kernel.org,
        antonio.gomez.iglesias@linux.intel.com, neelima.krishnan@intel.com,
        stable@vger.kernel.org
Subject: Re: [PATCH] x86/tsx: Use MSR_TSX_CTRL to clear CPUID bits
Message-ID: <20220214224121.ilhu23cfjdyhvahk@guptapa-mobl1.amr.corp.intel.com>
References: <5bd785a1d6ea0b572250add0c6617b4504bc24d1.1644440311.git.pawan.kumar.gupta@linux.intel.com>
 <YgqToxbGQluNHABF@zn.tnic>
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On 14.02.2022 18:38, Borislav Petkov wrote:
>On Wed, Feb 09, 2022 at 01:04:36PM -0800, Pawan Gupta wrote:
>> tsx_clear_cpuid() uses MSR_TSX_FORCE_ABORT to clear CPUID.RTM and
>> CPUID.HLE. Not all CPUs support MSR_TSX_FORCE_ABORT, alternatively use
>> MSR_IA32_TSX_CTRL when supported.
>>
>> Fixes: 293649307ef9 ("x86/tsx: Clear CPUID bits when TSX always force aborts")
>> Reported-by: kernel test robot <lkp@intel.com>
>> Tested-by: Neelima Krishnan <neelima.krishnan@intel.com>
>> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
>
><--- I'm assuming this needs to be
>
>Cc: <stable@vger.kernel.org>
>
>?

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.

>> @@ -106,13 +110,11 @@ void __init tsx_init(void)
>>  	int ret;
>>
>>  	/*
>> -	 * Hardware will always abort a TSX transaction if both CPUID bits
>> -	 * RTM_ALWAYS_ABORT and TSX_FORCE_ABORT are set. In this case, it is
>> -	 * better not to enumerate CPUID.RTM and CPUID.HLE bits. Clear them
>> -	 * here.
>> +	 * Hardware will always abort a TSX transaction when CPUID
>> +	 * 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) &&
>> -	    boot_cpu_has(X86_FEATURE_TSX_FORCE_ABORT)) {
>> +	if (boot_cpu_has(X86_FEATURE_RTM_ALWAYS_ABORT)) {
>
>So you test here X86_FEATURE_RTM_ALWAYS_ABORT and tsx_clear_cpuid()
>tests it again. Simplify I guess.

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. 

>>  		tsx_ctrl_state = TSX_CTRL_RTM_ALWAYS_ABORT;
>>  		tsx_clear_cpuid();
>>  		setup_clear_cpu_cap(X86_FEATURE_RTM);
>
>Also, here you clear X86_FEATURE_RTM and X86_FEATURE_HLE but the other
>caller of tsx_clear_cpuid() - init_intel() - doesn't clear those bits.
>Why?

Calling setup_clear_cpu_cap() by boot CPU is sufficient to clear these
bits for secondary CPUs. Moreover, init_intel() is also called during
cpu hotplug, clearing cached CPUID bits will not be safe after boot.

>IOW, can we concentrate the whole tsx_clear_cpuid() logic inside it and
>have callers only call it unconditionally. Then the decision whether
>to disable TSX and clear bits will happen all solely in that function
>instead of being spread around the boot code and being inconsistent.

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

Thanks,
Pawan