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[Qemu-devel] Re: [PATCH] rtl8139: IO memory is not part of vmstate


From: Michael S. Tsirkin
Subject: [Qemu-devel] Re: [PATCH] rtl8139: IO memory is not part of vmstate
Date: Wed, 15 Dec 2010 12:07:57 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Tue, Dec 14, 2010 at 08:41:55AM -0700, Alex Williamson wrote:
> On Tue, 2010-12-14 at 14:32 +0200, Michael S. Tsirkin wrote:
> > On Mon, Dec 13, 2010 at 10:00:48PM -0700, Alex Williamson wrote:
> > > On Tue, 2010-12-14 at 06:43 +0200, Michael S. Tsirkin wrote:
> > > > On Mon, Dec 13, 2010 at 12:15:08PM -0700, Alex Williamson wrote:
> > > > > On Mon, 2010-12-13 at 21:06 +0200, Michael S. Tsirkin wrote:
> > > > > > On Mon, Dec 13, 2010 at 11:59:16AM -0700, Alex Williamson wrote:
> > > > > > > On Mon, 2010-12-13 at 20:54 +0200, Michael S. Tsirkin wrote:
> > > > > > > > On Mon, Dec 13, 2010 at 11:00:44AM -0700, Alex Williamson wrote:
> > > > > > > > > On Mon, 2010-12-13 at 19:50 +0200, Michael S. Tsirkin wrote:
> > > > > > > > > > On Mon, Dec 13, 2010 at 10:43:22AM -0700, Alex Williamson 
> > > > > > > > > > wrote:
> > > > > > > > > > > So, unfortunately, I stand by my original patch.
> > > > > > > > > > 
> > > > > > > > > > What about the one that put -1 in saved index for a 
> > > > > > > > > > hotplugged device?
> > > > > > > > > 
> > > > > > > > > There are still examples that don't work even without hotplug 
> > > > > > > > > (example 2
> > > > > > > > > and example 3 after the reboot).  That hack limits the 
> > > > > > > > > damage, but still
> > > > > > > > > leaves a latent bug for reboot and doesn't address the 
> > > > > > > > > non-hotplug
> > > > > > > > > scenarios.  So, I don't think it's worthwhile to pursue, and 
> > > > > > > > > we
> > > > > > > > > shouldn't pretend we can use it to avoid bumping the 
> > > > > > > > > version_id.
> > > > > > > > > Thanks,
> > > > > > > > > 
> > > > > > > > > Alex
> > > > > > > > 
> > > > > > > > I guess when we bump it we tell users: migration is completely
> > > > > > > > borken to the old version, don't even try it.
> > > > > > > > 
> > > > > > > > Is there a way for libvirt to discover such incompatibilities
> > > > > > > > and avoid the migration?
> > > > > > > 
> > > > > > > I don't know if libvirt has a way to query this in advance.  If a
> > > > > > > migration is attempted, the target will report:
> > > > > > > 
> > > > > > > savevm: unsupported version 5 for '0000:00:03.0/rtl8139' v4
> > > > > > > 
> > > > > > > And the source will continue running.  We waste plenty of bits 
> > > > > > > getting
> > > > > > > to that point,
> > > > > > 
> > > > > > Yes, this happens after all of memory has been migrated.
> > > > > 
> > > > > Better late than never :^\
> > > > 
> > > > One other question: can we do the same by creating a new (empty)
> > > > section? As was discussed in the past this is easier for
> > > > downstreams to cherry-pick.
> > > 
> > > The only way I can think to do that would be to have a subsection that
> > > is always included, but saves no data.  That would force a failure on
> > > new->old migration, but I don't think it really matches the intended
> > > purpose of subsections and feels like it's adding cruft for no gain.
> > > Maybe I'm missing something.  Juan, is there any advantage to trapping
> > > this in a subsection?  Thanks,
> > > 
> > > Alex
> > 
> > Maybe in this particular case the advantage is minimal.
> > But it seems easier to stick to a rule of no more version
> > bumps than argue about each case.
> 
> Do we have such a rule?

I think it's easier to have a hard rule than start arguing whether
each change is a subsection or not for each patch.
What is the downside?

> If we have a subsection who's needed function
> is return 1, I think that's a good indication that it's not appropriate
> for a subsection and the end result is equivalent to bumping the main
> driver vmstate version.

So if you feel strongly about it, go ahead, I'm just concerned
we'll now need to argue version or subsection for each migration-related
change.

> It's convoluted to try to hide a one-way upgrade in a subsection.

It may be convoluted but I think it works better with downstreams.
When we'll bump the version again in the future, it
should be possible for a downstream to pick up
just the next change, and skip this one.

But that's a general comment. In this case, I expect all
downstream to be able to pick this bugfix with no real downsides.

>  Thanks,
> 
> Alex



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