[PATCHv5 16/16] clocksource: arch_timer: use virtual counters

[Catalin Marinas]

On Thu, Jan 31, 2013 at 12:15:39PM +0000, Mark Rutland wrote:
> 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 travel backwards. If we do have hyp mode, and might be a
> KVM host, we have to use the physical timers, and require CNTVOFF to be
> zero so as to have a consistent view of time between the physical timers
> and virtual counters.
> 
> Any code which may want to alter CNTVOFF (e.g. KVM) will need to ensure
> that it is zeroed when entering the host.
> 
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>

Acked-by: Catalin Marinas <catalin.marinas at arm.com>