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    SubjectRe: [PATCH] x86: enable Data Operand Independent Timing Mode
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    On 1/26/23 05:52, Jann Horn wrote:
    > On Wed, Jan 25, 2023 at 4:30 PM Dave Hansen <dave.hansen@intel.com> wrote:
    >> Translating from Intel-speak: Intel thinks that DOITM purely a way to
    >> make the CPU run slower if you haven't already written code specifically
    >> to mitigate timing side channels. All pain, no gain.
    >>
    >> The kernel as a whole is not written that way.
    >
    > The kernel as a whole also doesn't really use the FPU registers for
    > anything other than checksumming and cryptography and stuff like that
    > (it's disabled in the compiler flags because the FPU registers
    > normally contain userspace state that must not be clobbered). The
    > instructions listed on that Intel help page are all weird PM* and VP*
    > arithmetic instructions that can't be generated from C code in the
    > kernel (except for weird subsystems in which every function is only
    > callable in kernel-FPU-enabled mode and the compiler is set to enable
    > FPU instruction generation, by which I mean amdgpu).

    Maybe I'm totally missing something, but I thought the scope here was
    the "non-data operand independent timing behavior for the listed
    instructions" referenced here:

    > https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/best-practices/data-operand-independent-timing-isa-guidance.html

    where the "listed instructions" is this list:

    > https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/resources/data-operand-independent-timing-instructions.html

    For example, that includes XOR with the 0x31 and 0x81 opcodes which
    there are plenty of in the kernel.

    That's a bit wider scope than the crazy instructions like VPLZCNTD. The
    crazy instructions list that I _think_ you were grepping for is the
    "Instructions That May Exhibit MCDT Behavior". That's also a fun one,
    but it is more narrow than the DOITM list.

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