sf00191076: Fix IBRS synchronization

    UBUNTU: SAUCE: : x86/speculation: Fix the IBRS synchronization

    Ubuntu v4.4 kernel uses the in-house patches for IBRS. The backports
    still have some problems causing the IBRS status wrong when
    context-switching between the VM and host. For example, the IBRS would be
    mistakenly enabled in the host when the switching from a IBRS-enabled
    VM and that causes the performance overhead in the host. The other
    condition could also mistakenly disables the IBRS in host when
    contex-switching from the guest. And this could be considered a CVE.

    The detail different situations analysis:

    (host IBRS, guest IBRS)

    1). (1, 0)

    - Enter the VM with ibrs_enabled = 0, the host and guest value
      comparison will be wrong and the ibrs bit would not be
    _DISABLED_(The hostval and guestval are all zero when enhanced IBRS
    doesn't exist). Then, the _VM_ IBRS register will be mistakenly
    enabled.

    - Exit the VM with ibrs_enabled = 0, the host and guest value
      comparison will be wrong and the ibrs bit would not be
    _RE-ENABLED_(The hostval and guestval are all zero when enhanced IBRS
    doesn't exist). Then, the _host_ IBRS register will be mistakenly
    disabled.

    2). (0, 0)
    - work correctly.

    3). (0, 1)

    - Enter the VM with the ibrs_enabled = 1, the hostval != guestval, so,
      the guestval can be restored and work correctly.

    - Exit the VM with the ibrs_enabled = 1, the hostval != guestval, so,
      the hostval can be restored and work correctly.

    4). (1, 1)

    - Enter the VM with the ibrs_enabled = 1, however, the hostval !=
      guestval, then the guestval will be evaluated again and set to the
    MSR. This is the redundant work to set up the SPEC_CTRL MSR. But, the
    result is still correct.

    - Exit the VM with the ibrs_enabled = 1, however, the hostval !=
      guestval, then, the hostval will be restored with the value
    0(x86_spec_ctrl_base == 0 without the enhanced IBRS feature), the IBRS
    bit is mistakenly disabledand causes the host kernel vulnerability.

    Fixes: 4d8d3dbed275 ("UBUNTU: SAUCE: x86/bugs, KVM: Support the combination ...")
    Fixes: f676aa34b402 ("x86/kvm: add MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD ...")
    Signed-off-by: Gavin Guo <email address hidden>