Re: [Xen-devel] [PATCH v5 5/9] monitor: ARM SMC events

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Mon, Jun 6, 2016 at 10:38 AM, Julien Grall <julien.grall@xxxxxxx> wrote:
> Hi Tamas,
>
> On 06/06/16 17:14, Tamas K Lengyel wrote:
>>
>> On Mon, Jun 6, 2016 at 9:56 AM, Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
>> wrote:
>>>
>>> On Mon, Jun 6, 2016 at 9:54 AM, Julien Grall <julien.grall@xxxxxxx>
>>> wrote:
>>
>> So either way, I don't see a technical reason why Xen should silently
>> swallow any SMC trap if the vm_event user specifically asked them to
>> be forwarded. Other then it being odd that some ARM chips have varying
>> behavior regarding a subset of SMC instructions, it should not affect
>> when the vm_event user gets the events. If the user requests that it
>> wants to get notified any time an SMC is trapped to the VMM, it
>> should, regardless of whether that makes sense for "us". Depending on
>> the use-case of the user, indeed it may need extra information if it
>> wants to do emulation. If that need arises, the interface can easily
>> be extended to accommodate that usecase. We can also add a comment
>> saying that the forwarded events may also include ones with failed
>> condition checks depending on the CPU implementation. Also, it would
>> also be possible in the future to add a monitor configuration bit
>> where the user can specify if it wants the failed condition check SMCs
>> ignored by default or not. At this time however I want to start simple
>> and just forward all events, adding more bits and pieces only as
>> needed.
>
>
> We disagree on what is a "starting simple". It easier to relax than
> restricting a behavior later one.
>
> Even if we decide to add a bit to ignore some SMC in a later version of Xen,
> the introspection app will need to carry the burden mentioned in lengthly
> way on the previous mails because they may want to support older version of
> Xen.
>
> It would not be that difficult to provide a clean interface from beginning,
> which would allow to support more than your usecase.
>
> Anyway, I am not gonna ack this patch for the modification in
> arch/arm/traps.c because I don't think this is the right way to go. I will
> let Stefano deciding on this one.
>

Sounds good to me, my use-case is fine either way so ultimately it's
up to you guys.

Tamas

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