Re: [Xen-devel] [PATCH v2 00/25] arm/altp2m: Introducing altp2m to ARM.

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 03/08/16 13:13, Julien Grall wrote:
>
>
> On 03/08/16 13:00, Andrew Cooper wrote:
>> On 03/08/16 12:53, Julien Grall wrote:
>>> On 02/08/16 17:08, Andrew Cooper wrote:
>>>> On 02/08/16 08:34, Julien Grall wrote:
>>>>> Hi Andrew,
>>>>>
>>>>> On 02/08/2016 00:14, Andrew Cooper wrote:
>>>>>> On 01/08/2016 19:15, Julien Grall wrote:
>>>>>>> On 01/08/16 18:10, Sergej Proskurin wrote:
>>>>>>>>
>>>>>>>> Hello all,
>>>>>>>
>>>>>>> Hello Sergej,
>>>>>>>
>>>>>>>> The following patch series can be found on Github[0] and is part
>>>>>>>> of my
>>>>>>>> contribution to this year's Google Summer of Code (GSoC)[1]. My
>>>>>>>> project is
>>>>>>>> managed by the organization The Honeynet Project. As part of
>>>>>>>> GSoC, I
>>>>>>>> am being
>>>>>>>> supervised by the Xen developer Tamas K. Lengyel
>>>>>>>> <tamas@xxxxxxxxxxxxx>, George
>>>>>>>> D. Webster, and Steven Maresca.
>>>>>>>>
>>>>>>>> In this patch series, we provide an implementation of the altp2m
>>>>>>>> subsystem for
>>>>>>>> ARM. Our implementation is based on the altp2m subsystem for x86,
>>>>>>>> providing
>>>>>>>> additional --alternate-- views on the guest's physical memory by
>>>>>>>> means of the
>>>>>>>> ARM 2nd stage translation mechanism. The patches introduce new
>>>>>>>> HVMOPs
>>>>>>>> and
>>>>>>>> extend the p2m subsystem. Also, we extend libxl to support
>>>>>>>> altp2m on
>>>>>>>> ARM and
>>>>>>>> modify xen-access to test the suggested functionality.
>>>>>>>>
>>>>>>>> To be more precise, altp2m allows to create and switch to
>>>>>>>> additional
>>>>>>>> p2m views
>>>>>>>> (i.e. gfn to mfn mappings). These views can be manipulated and
>>>>>>>> activated as
>>>>>>>> will through the provided HVMOPs. In this way, the active guest
>>>>>>>> instance in
>>>>>>>> question can seamlessly proceed execution without noticing that
>>>>>>>> anything has
>>>>>>>> changed. The prime scope of application of altp2m is Virtual
>>>>>>>> Machine
>>>>>>>> Introspection, where guest systems are analyzed from the
>>>>>>>> outside of
>>>>>>>> the VM.
>>>>>>>>
>>>>>>>> Altp2m can be activated by means of the guest control parameter
>>>>>>>> "altp2m" on x86
>>>>>>>> and ARM architectures.  The altp2m functionality by default can
>>>>>>>> also
>>>>>>>> be used
>>>>>>>> from within the guest by design. For use-cases requiring purely
>>>>>>>> external access
>>>>>>>> to altp2m, a custom XSM policy is necessary on both x86 and ARM.
>>>>>>>
>>>>>>> As said on the previous version, altp2m operation *should not* be
>>>>>>> exposed to ARM guest. Any design written for x86 may not fit
>>>>>>> exactly
>>>>>>> for ARM (and vice versa), you will need to explain why you think we
>>>>>>> should follow the same pattern.
>>>>>>
>>>>>> Sorry, but I am going to step in here and disagree.  All the x86
>>>>>> justifications for altp2m being accessible to guests apply
>>>>>> equally to
>>>>>> ARM, as they are hardware independent.
>>>>>>
>>>>>> I realise you are maintainer, but the onus is on you to justify why
>>>>>> the
>>>>>> behaviour should be different between x86 and ARM, rather than
>>>>>> simply to
>>>>>> complain at it being the same.
>>>>>>
>>>>>> Naturally, technical issues about the details of the
>>>>>> implementation, or
>>>>>> the algorithms etc. are of course fine, but I don't see any
>>>>>> plausible
>>>>>> reason why ARM should purposefully different from x86 in terms of
>>>>>> available functionality, and several good reasons why it should
>>>>>> be the
>>>>>> same (least of all, feature parity across architectures).
>>>>>
>>>>> The question here, is how a guest could take advantage to access to
>>>>> altp2m on ARM today? Whilst on x86 a guest could be notified about
>>>>> memaccess change, this is not yet the case on ARM.
>>>>
>>>> Does ARM have anything like #VE whereby an in-guest entity can receive
>>>> notification of violations?
>>>
>>> I am not entirely sure what is exactly the #VE. From my understanding,
>>> it use to report stage 2 violation to the guest, right? If so, I am
>>> not aware of any.
>>
>> #VE is a newly specified CPU exception, precisely for reporting state 2
>> violations (in ARM terminology).  It works very much like a pagefault.
>
> Thank you for the explanation. We don't have any specific exception to
> report stage 2 (I guess EPT for x86 terminology) violations.

It is currently specific to Intel's EPT implementation, but there is
nothing in principle preventing AMD reusing the meaning for their NPT
implementation.

>
> If the guest physical address does not belong to an emulated device or
> does not have an associated host address, the hypervisor will inject a
> data/prefetch abort to the guest.

This is where x86 and ARM differ quite a bit.  For "areas which don't
exist", the default is to discard writes and reads return ~0, rather
than to raise a fault with the software.

>
> Those aborts contains a fault status. For now it is always the same
> fault: debug fault on AArch32 and address size fault on AArch64. I
> don't think we can re-use one of the fault (see ARM D7-1949 in DDI
> 0487A.j for the list of fault code) to behave as #VE.
>
> I guess the best would be an event channel for this purpose.

Agreed.  If there is no hardware way of doing this, a PV way with event
channels should work fine.

~Andrew

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