Re: Possible bug? DOM-U network stopped working after fatal error reported in DOM0

["Roger Pau Monné" <roger.pau@citrix.com>]

On Fri, Dec 31, 2021 at 10:47:57PM +0800, G.R. wrote:
> On Fri, Dec 31, 2021 at 2:52 AM Roger Pau Monné <roger.pau@citrix.com> wrote:
> >
> > On Thu, Dec 30, 2021 at 11:12:57PM +0800, G.R. wrote:
> > > On Thu, Dec 30, 2021 at 3:07 AM Roger Pau Monné <roger.pau@citrix.com> wrote:
> > > >
> > > > On Wed, Dec 29, 2021 at 11:27:50AM +0100, Roger Pau Monné wrote:
> > > > > On Wed, Dec 29, 2021 at 05:13:00PM +0800, G.R. wrote:
> > > > > > >
> > > > > > > I think this is hitting a KASSERT, could you paste the text printed as
> > > > > > > part of the panic (not just he backtrace)?
> > > > > > >
> > > > > > > Sorry this is taking a bit of time to solve.
> > > > > > >
> > > > > > > Thanks!
> > > > > > >
> > > > > > Sorry that I didn't make it clear in the first place.
> > > > > > It is the same cross boundary assertion.
> > > > >
> > > > > I see. After looking at the code it seems like sglist will coalesce
> > > > > contiguous physical ranges without taking page boundaries into
> > > > > account, which is not suitable for our purpose here. I guess I will
> > > > > either have to modify sglist, or switch to using bus_dma. The main
> > > > > problem with using bus_dma is that it will require bigger changes to
> > > > > netfront I think.
> > > >
> > > > I have a crappy patch to use bus_dma. It's not yet ready for upstream
> > > > but you might want to give it a try to see if it solves the cross page
> > > > boundary issues.
> > > >
> > > I think this version is better.
> >
> > Thanks for all the testing.
> >
> > > It fixed the mbuf cross boundary issue and allowed me to boot from one
> > > disk image successfully.
> >
> > It's good to know it seems to handle splitting mbufs fragments at page
> > boundaries correctly.
> >
> > > But seems like this patch is not stable enough yet and has its own
> > > issue -- memory is not properly released?
> >
> > I know. I've been working on improving it this morning and I'm
> > attaching an updated version below.
> >
> Good news.
> With this  new patch, the NAS domU can serve iSCSI disk without OOM
> panic, at least for a little while.
> I'm going to keep it up and running for a while to see if it's stable over time.

Thanks again for all the testing. Do you see any difference
performance wise?

> BTW, an irrelevant question:
> What's the current status of HVM domU on top of storage driver domain?
> About 7 years ago, one user on the list was able to get this setup up
> and running with your help (patch).[1]
> When I attempted to reproduce a similar setup two years later, I
> discovered that the patch was not submitted.
> And even with that patch the setup cannot be reproduced successfully.
> We spent some time debugging on the problem together[2], but didn't
> bottom out the root cause at that time.
> In case it's still broken and you still have the interest and time, I
> can launch a separate thread on this topic and provide required
> testing environment.

Yes, better as a new thread please.

FWIW, I haven't looked at this since a long time, but I recall some
fixes in order to be able to use driver domains with HVM guests, which
require attaching the disk to dom0 in order for the device model
(QEMU) to access it.

I would give it a try without using stubdomains and see what you get.
You will need to run `xl devd` inside of the driver domain, so you
will need to install xen-tools on the domU. There's an init script to
launch `xl devd` at boot, it's called 'xendriverdomain'.

Thanks, Roger.