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Re: [Qemu-devel] [RFC] save/restore with icount enabled.

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [RFC] save/restore with icount enabled.
Date: Wed, 15 May 2013 15:46:19 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130311 Thunderbird/17.0.4

Il 15/05/2013 15:35, KONRAD Frédéric ha scritto:
> On 15/05/2013 14:50, Paolo Bonzini wrote:
>> Il 15/05/2013 11:31, KONRAD Frédéric ha scritto:
>>> Hi,
>>> We are trying to do a simple save/restore on the VM with icount enabled.
>>> We saw that qemu_icount_bias and qemu_icount in cpus.c are not
>>> saved/restored,
>>> and icount_extra, icount_decr in CPUState neither, so the vm_clock is
>>> just
>>> growing normally after restoring the VM is that normal?
>>> We think that this is making the "replay" undeterminastic, which is
>>> bad for
>>> reverse execution.
>>> Is there a good reason for them not being saved?
>> No.  You can probably move icount to timers_state.  Something like
>> zeroing qemu_icount in cpu_disable_ticks() is required in order to save
>> qemu_icount_bias like the other members of timers_state.  There is even
>> a dummy field that you can reuse to avoid the pain of adding a
>> subsection to vmstate_timers. :)
>> Paolo
> Yes nice point :).
> What about icount_extra and icount_decr defined in CPU_COMMON? in
> cpu-defs.h?
>     int64_t icount_extra; /* Instructions until next timer event. */   \
>     /* Number of cycles left, with interrupt flag in high bit.          \
>        This allows a single read-compare-cbranch-write sequence to test \
>        for both decrementer underflow and exceptions. */               \
>     union {                                                             \
>         uint32_t u32;                                                   \
>         icount_decr_u16 u16;                                            \
>     } icount_decr;                                                      \

I'm not sure, I don't know that part well.  But IIRC they are recomputed
every time the VM restarts, so you don't need to save them?


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