• Hendrik Brueckner's avatar
    s390/kernel: lazy restore fpu registers · 9977e886
    Hendrik Brueckner authored
    Improve the save and restore behavior of FPU register contents to use the
    vector extension within the kernel.
    The kernel does not use floating-point or vector registers and, therefore,
    saving and restoring the FPU register contents are performed for handling
    signals or switching processes only.  To prepare for using vector
    instructions and vector registers within the kernel, enhance the save
    behavior and implement a lazy restore at return to user space from a
    system call or interrupt.
    To implement the lazy restore, the save_fpu_regs() sets a CPU information
    flag, CIF_FPU, to indicate that the FPU registers must be restored.
    Saving and setting CIF_FPU is performed in an atomic fashion to be
    interrupt-safe.  When the kernel wants to use the vector extension or
    wants to change the FPU register state for a task during signal handling,
    the save_fpu_regs() must be called first.  The CIF_FPU flag is also set at
    process switch.  At return to user space, the FPU state is restored.  In
    particular, the FPU state includes the floating-point or vector register
    contents, as well as, vector-enablement and floating-point control.  The
    FPU state restore and clearing CIF_FPU is also performed in an atomic
    For KVM, the restore of the FPU register state is performed when restoring
    the general-purpose guest registers before the SIE instructions is started.
    Because the path towards the SIE instruction is interruptible, the CIF_FPU
    flag must be checked again right before going into SIE.  If set, the guest
    registers must be reloaded again by re-entering the outer SIE loop.  This
    is the same behavior as if the SIE critical section is interrupted.
    Signed-off-by: default avatarHendrik Brueckner <brueckner@linux.vnet.ibm.com>
    Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
signal.c 15.8 KB