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Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase |
Date: |
Thu, 21 May 2015 10:47:46 +0100 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
* Kevin Wolf (address@hidden) wrote:
> Am 20.05.2015 um 14:07 hat Peter Maydell geschrieben:
> > On 20 May 2015 at 12:55, John Snow <address@hidden> wrote:
> > > So even if /currently/ we can reconstitute it from the register values,
> > > we may eventually be unable to.
> > >
> > > post_load will work for now, but I fear the case (in ten years) when
> > > someone else cleans up FDC code but fails to realize that the phase is
> > > not explicitly migrated.
> >
> > I assumed we would only do the post-load thing if the
> > source for migration doesn't migrate phase (however we
> > phrase that in a vmstate struct).
>
> I think if we extend the VMState, then we have two options. I'm not
> exactly sure how they work in details, but I'll try an educated guess -
> Juan, please correct me if I'm wrong:
>
> 1. We increase version_id and add the new field at the end. This breaks
> backwards migration; on forward migration the new field would be
> initialised with 0 and a post_load handler could check the old
> version_id to calculate the phase from register bits.
>
> 2. We add a subsection for the phase, and declare one phase to be the
> default (most likely the command phase) for which a subsection is not
> sent. In this case, the destination can't distinguish between a
> missing subsection because the source was running an old qemu or
> because it is the default phase. Unclear whether post_load should
> recalculate the phase or not.
There's another variant; add a subsection and always send it for new machine
types.
> If the above is correct, I'm afraid that the third option - which
> doesn't address John's (valid) concerns - would be the most reasonable:
>
> 3. Don't add any VMState fields now and just do the post_load handler.
> If we ever extend fdc in a way that makes it impossible to
> reconstruct the phase from other migrated state, we add a subsection
> that is only sent in cases where it differs from the reconstructed
> value.
Dave
>
> Kevin
>
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, (continued)
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, Peter Maydell, 2015/05/19
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, John Snow, 2015/05/19
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, Peter Maydell, 2015/05/19
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, Kevin Wolf, 2015/05/20
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, Peter Maydell, 2015/05/20
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, Kevin Wolf, 2015/05/20
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, Peter Maydell, 2015/05/20
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, John Snow, 2015/05/20
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, Peter Maydell, 2015/05/20
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, Kevin Wolf, 2015/05/21
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase,
Dr. David Alan Gilbert <=
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, Peter Maydell, 2015/05/21
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, Kevin Wolf, 2015/05/21
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, Markus Armbruster, 2015/05/21
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, Peter Maydell, 2015/05/21
- Re: [Qemu-block] [Qemu-devel] [PATCH 3/8] fdc: Introduce fdctrl->phase, Dr. David Alan Gilbert, 2015/05/21
[Qemu-block] [PATCH 5/8] fdc: Code cleanup in fdctrl_write_data(), Kevin Wolf, 2015/05/19
[Qemu-block] [PATCH 4/8] fdc: Use phase in fdctrl_write_data(), Kevin Wolf, 2015/05/19