Re: [Xen-devel] Linux 4.19.5 fails to boot as Xen dom0

["Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>]

On Thu, Nov 29, 2018 at 03:00:45PM +0000, Juergen Gross wrote:
> On 29/11/2018 15:32, Kirill A. Shutemov wrote:
> > On Thu, Nov 29, 2018 at 02:24:47PM +0000, Kirill A. Shutemov wrote:
> >> On Thu, Nov 29, 2018 at 01:35:17PM +0000, Juergen Gross wrote:
> >>> On 29/11/2018 14:26, Kirill A. Shutemov wrote:
> >>>> On Thu, Nov 29, 2018 at 09:41:25AM +0000, Juergen Gross wrote:
> >>>>> On 29/11/2018 02:22, Hans van Kranenburg wrote:
> >>>>>> Hi,
> >>>>>>
> >>>>>> As also seen at:
> >>>>>> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=914951
> >>>>>>
> >>>>>> Attached there are two serial console output logs. One is starting with
> >>>>>> Xen 4.11 (from debian unstable) as dom0, and the other one without Xen.
> >>>>>>
> >>>>>> [    2.085543] BUG: unable to handle kernel paging request at
> >>>>>> ffff888d9fffc000
> >>>>>> [    2.085610] PGD 200c067 P4D 200c067 PUD 0
> >>>>>> [    2.085674] Oops: 0000 [#1] SMP NOPTI
> >>>>>> [    2.085736] CPU: 1 PID: 1 Comm: swapper/0 Not tainted
> >>>>>> 4.19.0-trunk-amd64 #1 Debian 4.19.5-1~exp1+pvh1
> >>>>>> [    2.085823] Hardware name: HP ProLiant DL360 G7, BIOS P68 05/21/2018
> >>>>>> [    2.085895] RIP: e030:ptdump_walk_pgd_level_core+0x1fd/0x490
> >>>>>> [...]
> >>>>>
> >>>>> The offending stable commit is 4074ca7d8a1832921c865d250bbd08f3441b3657
> >>>>> ("x86/mm: Move LDT remap out of KASLR region on 5-level paging"), this
> >>>>> is commit d52888aa2753e3063a9d3a0c9f72f94aa9809c15 upstream.
> >>>>>
> >>>>> Current upstream kernel is booting fine under Xen, so in general the
> >>>>> patch should be fine. Using an upstream kernel built from above commit
> >>>>> (with the then needed Xen fixup patch 1457d8cf7664f34c4ba534) is fine,
> >>>>> too.
> >>>>>
> >>>>> Kirill, are you aware of any prerequisite patch from 4.20 which could be
> >>>>> missing in 4.19.5?
> >>>>
> >>>> I'm not.
> >>>>
> >>>> Let me look into this.
> >>>>
> >>>
> >>> What is making me suspicious is the failure happening just after
> >>> releasing the init memory. Maybe there is an access to .init.data
> >>> segment or similar? The native kernel booting could be related to the
> >>> usage of 2M mappings not being available in a PV-domain.
> >>
> >> Ahh.. Could you test this:
> >>
> >> diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
> >> index a12afff146d1..7dec63ec7aab 100644
> >> --- a/arch/x86/mm/dump_pagetables.c
> >> +++ b/arch/x86/mm/dump_pagetables.c
> >> @@ -496,7 +496,7 @@ static inline bool is_hypervisor_range(int idx)
> >>     * ffff800000000000 - ffff87ffffffffff is reserved for
> >>     * the hypervisor.
> >>     */
> >> -  return  (idx >= pgd_index(__PAGE_OFFSET) - 16) &&
> >> +  return  (idx >= pgd_index(__PAGE_OFFSET) - 17) &&
> >>            (idx <  pgd_index(__PAGE_OFFSET));
> >>  #else
> >>    return false;
> > 
> > Or, better, this:
> 
> That makes it boot again!
> 
> Any idea why upstream doesn't need it?

Nope.

I'll prepare a proper fix.

-- 
 Kirill A. Shutemov

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