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From: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
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Date: Tue, 26 Jan 2016 23:08:32 +0000
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Subject: Re: [Qemu-devel] Migrating decrementer
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To: David Gibson <david@gibson.dropbear.id.au>
Cc: quintela@redhat.com, Alexey Kardashevskiy <aik@ozlabs.ru>, agraf@suse.de, qemu-devel@nongnu.org, qemu-ppc@nongnu.org, amit.shah@redhat.com

On 26/01/16 22:31, Mark Cave-Ayland wrote:

> For a typical savevm/loadvm pair I see something like this:
> 
> savevm:
> 
> tb->guest_timebase = 26281306490558
> qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) = 7040725511
> 
> loadvm:
> 
> cpu_get_host_ticks() = 26289847005259
> tb_off_adj = -8540514701
> qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) = 7040725511
> cpu_ppc_get_tb() = -15785159386
> 
> But as cpu_ppc_get_tb() uses QEMU_CLOCK_VIRTUAL for vmclk we end up with
> a negative number for the timebase since the virtual clock is dwarfed by
> the number of TSC ticks calculated for tb_off_adj. This will work on a
> PPC host though since cpu_host_get_ticks() is also derived from the
> timebase.

Ah... the magic of signed numbers. Here's another attempt but this time
with cpu_ppc_get_tb() printed unsigned:

savevm:

tb->guest_timebase = 2597350923332
qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) = 4199081744
cpu_ppc_get_tb() = 69704756

loadvm:

cpu_get_host_ticks() = 2606380782824
tb_off_adj = -9029859492
qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) = 4199081744
cpu_ppc_get_tb() = 18446744064749396880


This implies that the cpu_ppc_get_tb() value post-migration is far too
low - presumably because tb_env->tb_freq is only 16MHz compared to the
actual TSC rate?


ATB,

Mark.