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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

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.

Thanks for investigating! Sure enough, my suspicion was correct - all it
takes to get that domain crash is to change the VM screen resolution.


Thanks,
Razvan

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