Re: [Qemu-devel] [PATCH v3 1/1] atapi: make change media detection for guests easier

[Pavel Hrdina]

On Mon, 2012-11-26 at 16:08 +0100, Kevin Wolf wrote:
> Am 26.11.2012 15:56, schrieb Pavel Hrdina:
> > On Mon, 2012-11-26 at 14:55 +0100, Kevin Wolf wrote:
> >> Am 26.11.2012 13:43, schrieb Pavel Hrdina:
> >>> On Fri, 2012-11-23 at 13:09 +0100, Kevin Wolf wrote:
> >>>> Another thing I would consider is using cdrom_changed = 0/1/2 instead of
> >>>> adding fake_cdrom_eject to add another state. Migration would
> >>>> automatically do the right thing for it, old versions would in both 1
> >>>> and 2 state switch to ASC_MEDIUM_MAY_HAVE_CHANGED next, which is correct
> >>>> in the latter case and is in the former case the same bug as the old
> >>>> qemu we're migrating to has anyway.
> >>>>
> >>>
> >>> I do it that way at first, but then I rewrite it, because I thought that
> >>> using new state would be better. But if you agree with cdrom_changed =
> >>> 0/1/2, I'll change it.
> >>
> >> I think it's nicer to have only one state. And if I'm not mistaken, we
> >> can even do without the pre_save/post_load handlers then.
> >>
> >> Kevin
> > 
> > I don't think that we can do it without the pre_save/post_load handlers,
> > because if the cdrom_changed could be set also to 2 then in case, that
> > you migrate immediately after the change from "buggy" qemu to the
> > "fixed" qemu we have in cdrom_changed state 1, but we need state 2. 
> 
> But you can't tell which is the right state anyway, because the buggy
> qemu on your source has only one "changed" state. Choosing state 1 seems
> fine to me. The worst thing that can happen is that you return
> ASC_MEDIUM_NOT_PRESENT twice - not a big deal.

At first I wanted to make state 1 as ASC_MEDIUM_MAY_HAVE_CHANGED and
state 2 as ASC_MEDIUM_NOT_PRESENT. In that case I would have to in
post_load and in ide_cd_change_cb set cdrom_changed to 2.

But if we switch these states, then only what we have to do is fix the
code in atapi.c. I'll sent the patch.

> 
> > And
> > otherwise, if you want to migrate from "fixed" qemu to "buggy" qemu
> > where version_id is lower then 3, you have to set the asc to
> > ASC_MEDIUM_MAY_HAVE_CHANGED.
> 
> This part is true. However, I'm pretty sure that migration from 1.3 to
> 0.11 doesn't work anyway, so why bother?

Yes, you're right.

Pavel

> 
> Kevin
>