Re: [BUG] XEN domU crash when PV grub chainloads 32-bit domU grub

[Ian Campbell <ian.campbell@citrix.com>]

On Thu, 2015-09-24 at 19:28 +0200, Andreas Sundstrom wrote:
> On 2015-09-23 16:18, Ian Campbell wrote:
> > On Wed, 2015-09-23 at 12:47 +0000, Andreas Sundstrom wrote:
> > > Citerar Ian Campbell <ian.campbell@citrix.com>:
> > > 
> > > > Along those lines, if the _host_ has buckets of RAM then might it
> > > > be
> > > > worth
> > > > restricting it in case the issue is with getting MFNs which are not
> > > > representably by the 32-bit PV interfaces? (IIRC the limit is ~160G
> > > > due
> > > > to
> > > > the size of the m2p hole, a 32-bit MFN spans 16TB so it's unlikely
> > > > to
> > > > be
> > > > that).
> > > > 
> > > > Likewise maybe the issue is with full addresses which don't fit in
> > > > a 32
> > > > -bit
> > > > number (which is maybe more likely to happen if grub uses a 1:1
> > > > mapping
> > > > like I would guess it does), so limiting the host to <4GB might
> > > > also be
> > > > interesting?
> > > > 
> > > 
> > > If this was meant for me I will need more information to understand  
> > > what to test.
> > > dom0 has either 12G or 8G memory in my test machines if that makes a 
> > > difference.
> > 
> > It was, sorry for not being clear.
> > 
> > How much memory do the test machines have?
> > 
> > If it is more than 160G then try booting with "mem=160G" on the
> > hypervisor
> > (not Linux) command line. You can just edit that in via grub.
> > 
> > Then try with mem=4G (which might require shrinking dom0 too of
> > course).
> 
> Well as I said my test machines only have 12 and 8G of memory.

You said dom0 did, from which I wasn't able to tell how much RAM the host
had, giving 12GB to dom0 on a 256G machine would be a plausible
configuration. But this is a confusing distinction for many and I should
have made that reasoning for including the 160G test clearer, sorry.

> I did a quick test with mem=2G though just to be sure, it failed on
> first attempt.

OK, so it is unlikely to be any of the possible integer overflow type
things I was thinking of then, thanks for testing.

Ian.