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

[Julien Grall <julien.grall@xxxxxxx>]

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.
Regards,

--
Julien Grall

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