lkml.org 
[lkml]   [2022]   [Feb]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    SubjectRe: [PATCH] x86/tsx: Use MSR_TSX_CTRL to clear CPUID bits
    Date
    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
    \
     
     \ /
      Last update: 2022-02-16 12:48    [W:2.500 / U:0.024 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site