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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Mon, 5 Oct 2015 16:09:52 +0200
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Subject: Re: [Qemu-devel] [PATCH] target-arm: Use common CPU cycle
 infrastructure
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To: Peter Maydell <peter.maydell@linaro.org>, Christopher Covington <cov@codeaurora.org>
Cc: Laurent Vivier <lvivier@redhat.com>, Peter Crosthwaite <crosthwaitepeter@gmail.com>, "qemu-devel@nongnu.org Developers" <qemu-devel@nongnu.org>, Alistair Francis <alistair.francis@xilinx.com>



On 03/10/2015 00:41, Peter Maydell wrote:
> > What I meant to ask was, do you see any reason for cpu_get_ticks() to exist?
> > If no architecture besides i386 wants to use it, perhaps the code should be
> > moved there.
>
> OTOH various non-x86 things do use the closely related cpu_get_real_ticks(),
> and the implementation of cpu_get_ticks() is very closely related to
> the other clock code in cpus.c.

cpu_get_real_ticks() is returning the host cycle counter;
cpu_get_ticks() is stopping/resuming it when the VM is stopped/resumed.
 In other words, cpu_get_real_ticks() is to cpu_get_ticks() what
QEMU_CLOCK_REALTIME is to QEMU_CLOCK_VIRTUAL.

They are also similar in that both cpu_get_ticks() and
QEMU_CLOCK_VIRTUAL "morph" to the instruction count in icount mode.

cpu_get_ticks() should be the right implementation for the ARM PMCCNTR
cycle counter, since: 1) PMCCNTR is roughly the same as the x86 RDTSC;
2) cpu_get_ticks() is the only monotonically increasing value outside
icount mode.

Paolo