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    Subject[PATCH v2 0/6] Delay VERW
    v2:
    - Removed the extra EXEC_VERW macro layers. (Sean)
    - Move NOPL before VERW. (Sean)
    - s/USER_CLEAR_CPU_BUFFERS/CLEAR_CPU_BUFFERS/. (Josh/Dave)
    - Removed the comments before CLEAR_CPU_BUFFERS. (Josh)
    - Remove CLEAR_CPU_BUFFERS from NMI returning to kernel and document the
    reason. (Josh/Dave)
    - Reformat comment in md_clear_update_mitigation(). (Josh)
    - Squash "x86/bugs: Cleanup mds_user_clear" patch. (Nikolay)
    - s/GUEST_CLEAR_CPU_BUFFERS/CLEAR_CPU_BUFFERS/. (Josh)
    - Added a patch from Sean to use CFLAGS.CF for VMLAUNCH/VMRESUME
    selection. This facilitates a single CLEAR_CPU_BUFFERS location for both
    VMLAUNCH and VMRESUME. (Sean)

    v1: https://lore.kernel.org/r/20231020-delay-verw-v1-0-cff54096326d@linux.intel.com

    Hi,

    Legacy instruction VERW was overloaded by some processors to clear
    micro-architectural CPU buffers as a mitigation of CPU bugs. This series
    moves VERW execution to a later point in exit-to-user path. This is
    needed because in some cases it may be possible for kernel data to be
    accessed after VERW in arch_exit_to_user_mode(). Such accesses may put
    data into MDS affected CPU buffers, for example:

    1. Kernel data accessed by an NMI between VERW and return-to-user can
    remain in CPU buffers (since NMI returning to kernel does not
    execute VERW to clear CPU buffers).
    2. Alyssa reported that after VERW is executed,
    CONFIG_GCC_PLUGIN_STACKLEAK=y scrubs the stack used by a system
    call. Memory accesses during stack scrubbing can move kernel stack
    contents into CPU buffers.
    3. When caller saved registers are restored after a return from
    function executing VERW, the kernel stack accesses can remain in
    CPU buffers(since they occur after VERW).

    Although these cases are less practical to exploit, moving VERW closer
    to ring transition reduces the attack surface.

    Overview of the series:

    Patch 1: Prepares VERW macros for use in asm.
    Patch 2: Adds macros to 64-bit entry/exit points.
    Patch 3: Adds macros to 32-bit entry/exit points.
    Patch 4: Enables the new macros.
    Patch 5: Uses CFLAGS.CF for VMLAUNCH/VMRESUME selection.
    Patch 6: Adds macro to VMenter.

    Below is some performance data collected with v1 on a Skylake client
    compared with previous implementation:

    Baseline: v6.6-rc5

    | Test | Configuration | Relative |
    | ------------------ | ---------------------- | -------- |
    | build-linux-kernel | defconfig | 1.00 |
    | hackbench | 32 - Process | 1.02 |
    | nginx | Short Connection - 500 | 1.01 |

    Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
    ---
    Pawan Gupta (5):
    x86/bugs: Add asm helpers for executing VERW
    x86/entry_64: Add VERW just before userspace transition
    x86/entry_32: Add VERW just before userspace transition
    x86/bugs: Use ALTERNATIVE() instead of mds_user_clear static key
    KVM: VMX: Move VERW closer to VMentry for MDS mitigation

    Sean Christopherson (1):
    KVM: VMX: Use BT+JNC, i.e. EFLAGS.CF to select VMRESUME vs. VMLAUNCH

    Documentation/arch/x86/mds.rst | 39 ++++++++++++++++++++++++++----------
    arch/x86/entry/entry_32.S | 3 +++
    arch/x86/entry/entry_64.S | 11 ++++++++++
    arch/x86/entry/entry_64_compat.S | 1 +
    arch/x86/include/asm/cpufeatures.h | 2 +-
    arch/x86/include/asm/entry-common.h | 1 -
    arch/x86/include/asm/nospec-branch.h | 31 +++++++++++++++++-----------
    arch/x86/kernel/cpu/bugs.c | 15 ++++++--------
    arch/x86/kernel/nmi.c | 2 --
    arch/x86/kvm/vmx/run_flags.h | 7 +++++--
    arch/x86/kvm/vmx/vmenter.S | 10 ++++++---
    arch/x86/kvm/vmx/vmx.c | 10 ++++++---
    12 files changed, 88 insertions(+), 44 deletions(-)
    ---
    base-commit: 05d3ef8bba77c1b5f98d941d8b2d4aeab8118ef1
    change-id: 20231011-delay-verw-d0474986b2c3

    Best regards,
    --
    Thanks,
    Pawan


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