• Martin Schwidefsky's avatar
    s390: remove all code using the access register mode · 0aaba41b
    Martin Schwidefsky authored
    The vdso code for the getcpu() and the clock_gettime() call use the access
    register mode to access the per-CPU vdso data page with the current code.
    
    An alternative to the complicated AR mode is to use the secondary space
    mode. This makes the vdso faster and quite a bit simpler. The downside is
    that the uaccess code has to be changed quite a bit.
    
    Which instructions are used depends on the machine and what kind of uaccess
    operation is requested. The instruction dictates which ASCE value needs
    to be loaded into %cr1 and %cr7.
    
    The different cases:
    
    * User copy with MVCOS for z10 and newer machines
      The MVCOS instruction can copy between the primary space (aka user) and
      the home space (aka kernel) directly. For set_fs(KERNEL_DS) the kernel
      ASCE is loaded into %cr1. For set_fs(USER_DS) the user space is already
      loaded in %cr1.
    
    * User copy with MVCP/MVCS for older machines
      To be able to execute the MVCP/MVCS instructions the kernel needs to
      switch to primary mode. The control register %cr1 has to be set to the
      kernel ASCE and %cr7 to either the kernel ASCE or the user ASCE dependent
      on set_fs(KERNEL_DS) vs set_fs(USER_DS).
    
    * Data access in the user address space for strnlen / futex
      To use "normal" instruction with data from the user address space the
      secondary space mode is used. The kernel needs to switch to primary mode,
      %cr1 has to contain the kernel ASCE and %cr7 either the user ASCE or the
      kernel ASCE, dependent on set_fs.
    
    To load a new value into %cr1 or %cr7 is an expensive operation, the kernel
    tries to be lazy about it. E.g. for multiple user copies in a row with
    MVCP/MVCS the replacement of the vdso ASCE in %cr7 with the user ASCE is
    done only once. On return to user space a CPU bit is checked that loads the
    vdso ASCE again.
    
    To enable and disable the data access via the secondary space two new
    functions are added, enable_sacf_uaccess and disable_sacf_uaccess. The fact
    that a context is in secondary space uaccess mode is stored in the
    mm_segment_t value for the task. The code of an interrupt may use set_fs
    as long as it returns to the previous state it got with get_fs with another
    call to set_fs. The code in finish_arch_post_lock_switch simply has to do a
    set_fs with the current mm_segment_t value for the task.
    
    For CPUs with MVCOS:
    
    CPU running in                        | %cr1 ASCE | %cr7 ASCE |
    --------------------------------------|-----------|-----------|
    user space                            |  user     |  vdso     |
    kernel, USER_DS, normal-mode          |  user     |  vdso     |
    kernel, USER_DS, normal-mode, lazy    |  user     |  user     |
    kernel, USER_DS, sacf-mode            |  kernel   |  user     |
    kernel, KERNEL_DS, normal-mode        |  kernel   |  vdso     |
    kernel, KERNEL_DS, normal-mode, lazy  |  kernel   |  kernel   |
    kernel, KERNEL_DS, sacf-mode          |  kernel   |  kernel   |
    
    For CPUs without MVCOS:
    
    CPU running in                        | %cr1 ASCE | %cr7 ASCE |
    --------------------------------------|-----------|-----------|
    user space                            |  user     |  vdso     |
    kernel, USER_DS, normal-mode          |  user     |  vdso     |
    kernel, USER_DS, normal-mode lazy     |  kernel   |  user     |
    kernel, USER_DS, sacf-mode            |  kernel   |  user     |
    kernel, KERNEL_DS, normal-mode        |  kernel   |  vdso     |
    kernel, KERNEL_DS, normal-mode, lazy  |  kernel   |  kernel   |
    kernel, KERNEL_DS, sacf-mode          |  kernel   |  kernel   |
    
    The lines with "lazy" refer to the state after a copy via the secondary
    space with a delayed reload of %cr1 and %cr7.
    
    There are three hardware address spaces that can cause a DAT exception,
    primary, secondary and home space. The exception can be related to
    four different fault types: user space fault, vdso fault, kernel fault,
    and the gmap faults.
    
    Dependent on the set_fs state and normal vs. sacf mode there are a number
    of fault combinations:
    
    1) user address space fault via the primary ASCE
    2) gmap address space fault via the primary ASCE
    3) kernel address space fault via the primary ASCE for machines with
       MVCOS and set_fs(KERNEL_DS)
    4) vdso address space faults via the secondary ASCE with an invalid
       address while running in secondary space in problem state
    5) user address space fault via the secondary ASCE for user-copy
       based on the secondary space mode, e.g. futex_ops or strnlen_user
    6) kernel address space fault via the secondary ASCE for user-copy
       with secondary space mode with set_fs(KERNEL_DS)
    7) kernel address space fault via the primary ASCE for user-copy
       with secondary space mode with set_fs(USER_DS) on machines without
       MVCOS.
    8) kernel address space fault via the home space ASCE
    
    Replace user_space_fault() with a new function get_fault_type() that
    can distinguish all four different fault types.
    
    With these changes the futex atomic ops from the kernel and the
    strnlen_user will get a little bit slower, as well as the old style
    uaccess with MVCP/MVCS. All user accesses based on MVCOS will be as
    fast as before. On the positive side, the user space vdso code is a
    lot faster and Linux ceases to use the complicated AR mode.
    Reviewed-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
    Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
    Signed-off-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
    0aaba41b
lowcore.h 6.14 KB