• Mark Rutland's avatar
    clocksource: arch_timer: use virtual counters · 714c21cb
    Mark Rutland authored
    commit 0d651e4e65e96989f72236bf83bd4c6e55eb6ce4 upstream.
    
    Switching between reading the virtual or physical counters is
    problematic, as some core code wants a view of time before we're fully
    set up. Using a function pointer and switching the source after the
    first read can make time appear to go backwards, and having a check in
    the read function is an unfortunate block on what we want to be a fast
    path.
    
    Instead, this patch makes us always use the virtual counters. If we're a
    guest, or don't have hyp mode, we'll use the virtual timers, and as such
    don't care about CNTVOFF as long as it doesn't change in such a way as
    to make time appear to travel backwards. As the guest will use the
    virtual timers, a (potential) KVM host must use the physical timers
    (which can wake up the host even if they fire while a guest is
    executing), and hence a host must have CNTVOFF set to zero so as to have
    a consistent view of time between the physical timers and virtual
    counters.
    Signed-off-by: 's avatarMark Rutland <mark.rutland@arm.com>
    Acked-by: 's avatarCatalin Marinas <catalin.marinas@arm.com>
    Acked-by: 's avatarMarc Zyngier <marc.zyngier@arm.com>
    Acked-by: 's avatarSantosh Shilimkar <santosh.shilimkar@ti.com>
    Cc: Rob Herring <rob.herring@calxeda.com>
    Cc: Mark Brown <broonie@kernel.org>
    Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    714c21cb
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