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

[Andrew Cooper <Andrew.Cooper3@citrix.com>]

On 16/02/2022 01:28, Pawan Gupta wrote:
> 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.

Well for one, when Linux is starting up as the kexec environment
following Xen.

You're not coding for "what logic should follow a clean microcode
load".  You're coding for "how to take the arbitrary state that my
preceding environment left, and turn it into what I want".

Look no further than linuxboot for an environment where your bootloader
has already altered these settings.

~Andrew