linux-kernel - Re: [PATCH] x86/tsx: Use MSR_TSX

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Date:   Tue, 15 Feb 2022 17:28:41 -0800
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Andrew Cooper <Andrew.Cooper3@...rix.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "antonio.gomez.iglesias@...ux.intel.com" 
        <antonio.gomez.iglesias@...ux.intel.com>,
        "neelima.krishnan@...el.com" <neelima.krishnan@...el.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] x86/tsx: Use MSR_TSX_CTRL to clear CPUID bits

On 16.02.2022 00:49, Andrew Cooper wrote:
>On 16/02/2022 00:39, Pawan Gupta wrote:
>> On 15.02.2022 20:33, Borislav Petkov wrote:
>>> On Tue, Feb 15, 2022 at 10:19:31AM -0800, Pawan Gupta wrote:
>>>> I admit it has gotten complicated with so many bits associated with
>>>> TSX.
>>>
>>> Yah, and looka here:
>>>
>>> https://github.com/andyhhp/xen/commit/ad9f7c3b2e0df38ad6d54f4769d4dccf765fbcee
>>>
>>>
>>> It seems it isn't complicated enough. ;-\
>>>
>>> Andy just made me aware of this thing where you guys have added a new
>>> MSR bit:
>>>
>>> MSR_MCU_OPT_CTRL[1] which is called something like
>>> MCU_OPT_CTRL_RTM_ALLOW or so.
>>
>> RTM_ALLOW bit was added to MSR_MCU_OPT_CTRL, but its not set by default,
>> and it is *not* recommended to be used in production deployments [1]:
>>
>>   Although MSR 0x122 (TSX_CTRL) and MSR 0x123 (IA32_MCU_OPT_CTRL) can be
>>   used to reenable Intel TSX for development, doing so is not recommended
>>   for production deployments. In particular, applying MD_CLEAR flows for
>>   mitigation of the Intel TSX Asynchronous Abort (TAA) transient
>> execution
>>   attack may not be effective on these processors when Intel TSX is
>>   enabled with updated microcode. The processors continue to be mitigated
>>   against TAA when Intel TSX is disabled.
>
>The purpose of setting RTM_ALLOW isn't to enable TSX per say.
>
>The purpose is to make MSR_TSX_CTRL.RTM_DISABLE behaves consistently on
>all hardware, which reduces the complexity and invasiveness of dealing
>with this special case, because the TAA workaround will still turn TSX
>off by default.
>
>The configuration you don't want to be running with is RTM_ALLOW &&
>!RTM_DISABLE, because that is "still vulnerable to TSX Async Abort".

I am not sure how a system can end up with RTM_ALLOW && !RTM_DISABLE? I
have no problem taking care of this case, I am just debating why we need
it.

One way to get to this state is BIOS sets RTM_ALLOW (dont know why?) and
linux cmdline has tsx=on.

Thanks,
Pawan