Message-ID: <437A4093.7050306@atlas.sk>
Date: Tue, 15 Nov 2005 21:09:55 +0100
From: ace <acelists@atlas.sk>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] [patch] fix QEMU hang after hibernate
References: <4349FB21.9040205@stanfordalumni.org>
In-Reply-To: <4349FB21.9040205@stanfordalumni.org>
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Reply-To: qemu-devel@nongnu.org
To: qemu-devel@nongnu.org

I can confirm this patch applied alone to 0.7.2 did something for me. I
suspended the PC for 1 minute (just to reboot the machine) and after
resume, my quest win95 was still working. I will of course try longer
periods later.

Peter

John Coiner wrote:

> 
> After I hibernate my x86 linux host machine (using the "swsusp2"
> package) and resume, QEMU and guest hang. That is odd, because
> hibernation is transparent to all other user programs.
> 
> Has anyone else noticed this? If so please try the patch.
> 
> You know QEMU is doing something weird, for it to be sensitive to
> hibernation. Indeed, the cause is something you don't see every day:
> 
> QEMU implements timers partly by reading the CPU's tick counter directly
> from hardware, with an architecture-specific assembly instruction
> ('rdtsc' on intel, and so on for other archs) rather than using purely
> OS-provided features to get the time.
> 
> The problem is that following hibernation, the CPU tick counter isn't
> what it used to be. On my host, it has a lower value -- it jumped
> backwards. This breaks QEMU's timers. Some of them decide to block for a
> very long time, which causes the guest to hang or behave oddly.
> 
> (Maybe this is technically a bug in "swsusp2", maybe it should restore
> the CPU tick counter to the previous value upon resume. But who cares.
> It's easy to fix this in QEMU, and it would not be easy to check and fix
> all hibernation schemes on all architectures and operating systems.)
> 
> The following patch fixes the 'cpu_get_ticks()' function to always
> return a non-decreasing value, even if the value read from hardware
> decreases. Hope it helps. Feedback is welcome.
> 
> -- John
> 
> 
> 
> --- qemu-0.7.2-dmapatch/vl.c    2005-09-04 13:11:31.000000000 -0400
> +++ qemu-0.7.2-broken/vl.c      2005-10-10 00:54:08.000000000 -0400
> @@ -545,14 +547,21 @@
>  #error unsupported CPU
>  #endif
> 
> -static int64_t cpu_ticks_offset;
> -static int cpu_ticks_enabled;
> +static int64_t cpu_ticks_prev = 0;
> +static int64_t cpu_ticks_offset = 0;
> +static int cpu_ticks_enabled = 0;
> 
>  static inline int64_t cpu_get_ticks(void)
>  {
>      if (!cpu_ticks_enabled) {
>          return cpu_ticks_offset;
>      } else {
> +       int64_t ticks = cpu_get_real_ticks();
> +       if( cpu_ticks_prev > ticks )
> +       {
> +           cpu_ticks_offset += ( cpu_ticks_prev - ticks );
> +       }
> +       cpu_ticks_prev = ticks;
>          return cpu_get_real_ticks() + cpu_ticks_offset;
>      }
>  }
> 
> 
> 
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> 
> 
> 
> 
>