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

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

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).


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