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

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

On Wed, 2015-09-23 at 00:37 +0200, Samuel Thibault wrote:
> Andreas Sundstrom, le Mon 21 Sep 2015 22:03:22 +0200, a écrit :
> > Note that my original thought was that this bug probably is within
> > GRUB.
> 
> It's probably in the GRUB implementation of loading the domU GRUB, since
> you say that pvgrub1 loads it fine.
> 
> > (XEN) domain_crash_sync called from entry.S: fault at ffff82d08021feb0
> > compat_create_bounce_frame+0xc6/0xde
> 
> So it's inside xen entry code...
> 
> > (XEN) Guest stack trace from esp=005a5ff0:
> 
> This looks like the end of the stack
> 
> > (XEN) 00000010 00000000 0001e019 00010046 0016b38b 0016b38a 0016b389
> > 0016b388
> > (XEN) 0016b387 0016b386 0016b385 0016b384 0016b383 0016b382 0016b381
> > 0016b380
> [...]
> 
> and this looks like a lot of consecutive numbers.  Perhaps we simply
> somehow overflow?  Did you try giving less memory to the domU?

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?

Ian.

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