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Re: [Xen-devel] [RFC v2 3/6] xen/arm: Allow platform_hvc to handle guest SMC calls

On Thu, Feb 9, 2017 at 1:01 PM, Edgar E. Iglesias
<edgar.iglesias@xxxxxxxxxx> wrote:
> On Thu, Feb 09, 2017 at 12:42:09PM -0700, Tamas K Lengyel wrote:
>> On Thu, Feb 9, 2017 at 11:39 AM, Julien Grall <julien.grall@xxxxxxx> wrote:
>> > Hi Tamas,
>> >
>> > On 02/09/2017 06:11 PM, Tamas K Lengyel wrote:
>> >>
>> >> On Wed, Feb 8, 2017 at 5:08 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
>> >>>
>> >>> On 08/02/2017 23:28, Tamas K Lengyel wrote:
>> >>>>
>> >>>> On Wed, Feb 8, 2017 at 3:04 PM, Julien Grall <julien.grall@xxxxxxx>
>> >>>> wrote:
>> >>>
>> >>> You haven't understood my point. Xen is currently emulating PSCI call for
>> >>> the guest to allow powering up and down the CPUs and other stuff. If you
>> >>> decide to trap all the SMCs, you would have to handle them.
>> >>
>> >>
>> >> Sure, it's more work on the monitor side, but other then that, what's
>> >> the problem?
>> >
>> >
>> > Because you will have to introduce hypercalls to get all the necessary
>> > information from Xen that will not be available from outside.
>> > Given that SMC has been designed to target different services (PSCI, 
>> > Trusted
>> > OS...) it would be normal to have monitor app only monitoring a certain set
>> > of SMC call. You cannot deny a such use case as it would avoid an monitor
>> > app to handle every single call that would be consumed by XEN but not
>> > forwarded to the secure firmware.
>> >
>> I have nothing against introducing a fine-tune option to the SMC
>> monitoring system so the monitor app can determine if it wants all
>> SMCs or only a subset. At the moment I don't know of any usecase that
>> would require this option. I certainly don't need it. If this option
>> gets implemented by someone, I would be happy to take it.
> Well, the reason it would be useful is the other way around.
> On for example ZynqMP, enabling the monitor is useless since it will
> turn off the ability to use the vital FW APIs needed for device
> passthrough.
> So the monitor only works for PV guests that call SMC APIs to some
> imaginary Firmware.
> While a monitor that didn't insist in consuming all SMC calls,
> could very well be useful for monitoring/tracing purposes or
> other stuff even with guests that actually use a "real" FW API.
> I don't think we need to implement support for the latter right away,
> we can incrementally add support for these things (I hope).

Certainly, as I said I have nothing against adding such a feature. All
I'm saying is that I don't know of any usecase that requires that
option at the moment, so I would be OK with just making the two
exclusive. If someone finds the time to implement such fine-tuning,
I'm all for it.


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