• Alex Smith's avatar
    MIPS: VDSO: Add implementations of gettimeofday() and clock_gettime() · a7f4df4e
    Alex Smith authored
    Add user-mode implementations of gettimeofday() and clock_gettime() to
    the VDSO. This is currently usable with 2 clocksources: the CP0 count
    register, which is accessible to user-mode via RDHWR on R2 and later
    cores, or the MIPS Global Interrupt Controller (GIC) timer, which
    provides a "user-mode visible" section containing a mirror of its
    counter registers. This section must be mapped into user memory, which
    is done below the VDSO data page.
    When a supported clocksource is not in use, the VDSO functions will
    return -ENOSYS, which causes libc to fall back on the standard syscall
    When support for neither of these clocksources is compiled into the
    kernel at all, the VDSO still provides clock_gettime(), as the coarse
    realtime/monotonic clocks can still be implemented. However,
    gettimeofday() is not provided in this case as nothing can be done
    without a suitable clocksource. This causes the symbol lookup to fail
    in libc and it will then always use the standard syscall path.
    This patch includes a workaround for a bug in QEMU which results in
    RDHWR on the CP0 count register always returning a constant (incorrect)
    value. A fix for this has been submitted, and the workaround can be
    removed after the fix has been in stable releases for a reasonable
    amount of time.
    A simple performance test which calls gettimeofday() 1000 times in a
    loop and calculates the average execution time gives the following
    results on a Malta + I6400 (running at 20MHz):
     - Syscall:    ~31000 ns
     - VDSO (GIC): ~15000 ns
     - VDSO (CP0): ~9500 ns
    - Minor code re-arrangements in order for mappings to be made
    in the order they appear to the process' address space.
    - Move do_{monotonic, realtime} outside of the MIPS_CLOCK_VSYSCALL ifdef
    - Use gic_get_usm_range so we can do the GIC mapping in the
    arch/mips/kernel/vdso instead of the GIC irqchip driver]
    Signed-off-by: default avatarAlex Smith <alex.smith@imgtec.com>
    Signed-off-by: default avatarMarkos Chandras <markos.chandras@imgtec.com>
    Cc: linux-kernel@vger.kernel.org
    Cc: linux-mips@linux-mips.org
    Patchwork: https://patchwork.linux-mips.org/patch/11338/Signed-off-by: default avatarRalf Baechle <ralf@linux-mips.org>