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

[Hans van Kranenburg <hans@xxxxxxxxxxx>]

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

Hans

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