Re: [Xen-devel] Weird altp2m behaviour when switching early to a new view

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 04/17/2018 01:49 PM, Razvan Cojocaru wrote:
> On 04/17/2018 11:24 AM, Razvan Cojocaru wrote:
>> On 04/16/2018 11:21 PM, George Dunlap wrote:
>>> On Mon, Apr 16, 2018 at 7:46 PM, Razvan Cojocaru
>>> <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>>> On 04/16/2018 08:47 PM, George Dunlap wrote:
>>>>> On 04/13/2018 03:44 PM, Razvan Cojocaru wrote:
>>>>>> On 04/11/2018 11:04 AM, Razvan Cojocaru wrote:
>>>>>>> Debugging continues.
>>>>>>
>>>>>> Finally, the attached patch seems to get the display unstuck in my
>>>>>> scenario, although for one guest I get:
>>>>>>
>>>>>> (XEN) d2v0 Unexpected vmexit: reason 49
>>>>>> (XEN) domain_crash called from vmx.c:4120
>>>>>> (XEN) Domain 2 (vcpu#0) crashed on cpu#1:
>>>>>> (XEN) ----[ Xen-4.11-unstable  x86_64  debug=y   Not tainted ]----
>>>>>> (XEN) CPU:    1
>>>>>> (XEN) RIP:    0010:[<fffff96000842354>]
>>>>>> (XEN) RFLAGS: 0000000000010246   CONTEXT: hvm guest (d2v0)
>>>>>> (XEN) rax: fffff88003000000   rbx: fffff900c0083db0   rcx: 
>>>>>> 00000000aa55aa55
>>>>>> (XEN) rdx: fffffa80041bdc41   rsi: fffff900c00c69a0   rdi: 
>>>>>> 0000000000000001
>>>>>> (XEN) rbp: 0000000000000000   rsp: fffff88002ee9ef0   r8:  
>>>>>> fffffa80041bdc40
>>>>>> (XEN) r9:  fffff80001810e80   r10: fffffa800342aa70   r11: 
>>>>>> fffff88002ee9e80
>>>>>> (XEN) r12: 0000000000000005   r13: 0000000000000001   r14: 
>>>>>> fffff900c00c08b0
>>>>>> (XEN) r15: 0000000000000001   cr0: 0000000080050031   cr4: 
>>>>>> 00000000000406f8
>>>>>> (XEN) cr3: 00000000ef771000   cr2: fffff900c00c8000
>>>>>> (XEN) fsb: 00000000fffde000   gsb: fffff80001810d00   gss: 
>>>>>> 000007fffffdc000
>>>>>> (XEN) ds: 002b   es: 002b   fs: 0053   gs: 002b   ss: 0018   cs: 0010
>>>>>>
>>>>>> i.e. EXIT_REASON_EPT_MISCONFIG - so not of the woods yet. I am hoping
>>>>>> somebody more familiar with the code can point to a more elegant
>>>>>> solution if one exists.
>>>>>
>>>>> I think I have an idea what's going on, but it's complicated. :-)
>>>>>
>>>>> Basically, the logdirty functionality isn't simple, and needs careful
>>>>> thought on how to integrate it.  I'll write some more tomorrow, and see
>>>>> if I can come up with a solution.
>>>>
>>>> I think I know why this happens for the one guest - the other guests
>>>> start at a certain resolution display-wise and stay that way until 
>>>> shutdown.
>>>>
>>>> This particular guest starts with a larger screen, then goes to roughly
>>>> 2/3rds of it, then tries to go back to the initial larger one - at which
>>>> point the above happens. I assume this corresponds to some pages being
>>>> removed and/or added. I'll test this theory more tomorrow - if it's
>>>> correct I should be able to reproduce the crash (with the patch) by
>>>> simply resetting the screen resolution (increasing it).
>>>
>>> The trick is that p2m_change_type doesn't actually iterate over the
>>> entire p2m range, individually changing entries as it goes.  Instead
>>> it misconfigures the entries at the top-level, which causes the kinds
>>> of faults shown above.  As it gets faults for each entry, it checks
>>> the current type, the logdirty ranges, and the global logdirty bit to
>>> determine what the new types should be.
>>>
>>> Your patch makes it so that all the altp2ms now get the
>>> misconfiguration when the logdirty range is changed; but clearly
>>> handling the misconfiguration isn't integrated properly with the
>>> altp2m system yet.  Doing it right may take some thought.
>>
>> FWIW, the attached patch has solved the misconfig-related domain crash
>> for me (though I'm very likely missing some subtleties). It all seems to
>> work as expected when enabling altp2m and switching early to a new view.
>> However, now I have domUs with a frozen display when I disconnect the
>> introspection application (that is, after I switch back to the default
>> view and disable altp2m on the domain).
> 
> The for() loop in the previous patch is unnecessary, so here's a new
> (cleaner) patch. I can't get the guest to freeze the display when
> detaching anymore - unrelated to the for() - (so it might have been
> something else in my setup), but I'll watch for it in the following days.
> 
> Hopefully this is either a reasonable fix or a basis for one.

Apologies, forgot to actually attach the patch. Here it is.

Attachment: altp2m_logdirty3.patch
Description: Text Data

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