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

[Hans van Kranenburg <hans@xxxxxxxxxxx>]

On 11/30/18 2:26 PM, Kirill A. Shutemov wrote:
> On Fri, Nov 30, 2018 at 01:11:56PM +0000, Hans van Kranenburg wrote:
>> On 11/29/18 4:06 PM, Kirill A. Shutemov wrote:
>>> 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.
>>>
>>
>> Thanks for looking into this.
>>
>> In the meantime, I applied the "Or, better, this" change, and my dom0
>> boots again.
>>
>> FYI, boot log now: (paste 90d valid)
>> https://paste.debian.net/plainh/48940826
> 
> I forgot to CC you:
> 
> https://lkml.kernel.org/r/20181130121131.g3xvlvixv7mvlr7b@xxxxxxxxxxxxxxxxxx
> 
> Please give it a try.

I'm not in that thread, so my response here...

You paste a v2-like patch into 'Re: [PATCH 1/2]'. Juergen says:
s/LDT_PGD_ENTRY/GUARD_HOLE_PGD_ENTRY/, then you say Ughh.., change it to
GUARD_HOLE_ENTRY, which does not exist, and then get a Reviewed-by from
Juergen.

I guess it has to be GUARD_HOLE_PGD_ENTRY after all...

arch/x86/include/asm/pgtable_64_types.h:116:31: error:
'GUARD_HOLE_ENTRY' undeclared (first use in this function); did you mean
'GUARD_HOLE_PGD_ENTRY'?

I'll test that instead.

Hans

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