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From: "Pavel Dovgalyuk" <dovgaluk@ispras.ru>
To: "'Dr. David Alan Gilbert'" <dgilbert@redhat.com>
Cc: 'Pavel Dovgalyuk' <Pavel.Dovgaluk@ispras.ru>,
	qemu-devel@nongnu.org, quintela@redhat.com, mst@redhat.com,
	maria.klimushenkova@ispras.ru, pbonzini@redhat.com
Subject: Re: [Qemu-devel] [ PATCH  ] hpet: recover timer offset correctly
Date: Wed, 20 Dec 2017 13:02:46 +0300	[thread overview]
Message-ID: <000a01d37979$af4f2640$0ded72c0$@ru> (raw)
In-Reply-To: <20171220094036.GB2348@work-vm>



> * Pavel Dovgalyuk (dovgaluk@ispras.ru) wrote:
> > > From: Dr. David Alan Gilbert [mailto:dgilbert@redhat.com]
> > > * Pavel Dovgalyuk (Pavel.Dovgaluk@ispras.ru) wrote:
> > > > HPET saves its state by calculating the current time and recovers timer
> > > > offset using this calculated value. But these calculations include
> > > > divisions and multiplications. Therefore the timer state cannot be recovered
> > > > precise enough.
> > > > This patch introduces saving of the original value of the offset to
> > > > preserve the determinism of the timer.
> > >
> > > Please put the extra data in a subsection and tie the subsection to a
> > > property enabled by the machine type.  That avoids breaking backwards
> > > migration compatibility.
> >
> > I thought about it and haven't found how to preserve the backward migration compatibility
> > without breaking everything else.
> > Loading old (or compatible) snapshot without changing the version will corrupt the offset in
> > post_load.
> 
> There are lots of ways; for example you could initialise hpet_offset to
> a dummy value in a pre-load and spot that in the post-load instead of
> the version_id, or set a flag in a post-load on the subsection.

Thanks. Please check the second version.

Pavel Dovgalyuk

      reply	other threads:[~2017-12-20 10:02 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-12-20  7:58 [Qemu-devel] [ PATCH ] hpet: recover timer offset correctly Pavel Dovgalyuk
2017-12-20  9:03 ` Paolo Bonzini
2017-12-20  9:17 ` Dr. David Alan Gilbert
2017-12-20  9:21   ` Pavel Dovgalyuk
2017-12-20  9:40     ` Dr. David Alan Gilbert
2017-12-20 10:02       ` Pavel Dovgalyuk [this message]

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