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

On Wed, Feb 08, 2017 at 10:04:53PM +0000, Julien Grall wrote:
> Hi Tamas,
> Can you please try to configure your e-mail client to use '>' rather than '
> '? It makes quite hard to read the e-mail.
> On 08/02/2017 20:15, Tamas K Lengyel wrote:
> >
> >
> >On Wed, Feb 8, 2017 at 12:40 PM, Edgar E. Iglesias
> ><edgar.iglesias@xxxxxxxxx <mailto:edgar.iglesias@xxxxxxxxx>> wrote:
> >    On Wed, Feb 08, 2017 at 06:29:13PM +0100, Edgar E. Iglesias wrote:
> >    If platform_hvc() consumes an SMC, it's considered part of the Xen API.
> >    Visible but not filterable by a monitor.
> >
> >
> >    Platforms can then dictate which SMC calls are better handled within
> >    Xen and
> >    which ones can be exposed to dom0 user-space.
> >
> >    In addition, there could be a hypercall to disable platform specific
> >    handling
> >    in Xen alltogether for a given guest. Then everything goes to dom0
> >    user-space.
> >
> >    It's a little messy...
> >
> >
> >
> >That is messy and I would not want any SMCs reaching the firmware when
> >the monitor application is in use. The monitor interface is disabled by
> >default and there aren't any known usecases where the SMC has to reach
> >both the firmware and the monitor application as well. So I think it is
> >safe to just make the two things mutually exclusive.
> If you look at the SMC Calling Convention [1] both HVC and SMC are
> considered a conduit for service call to the secure firmware or hypervisor.
> It would be up to the hypervisor deciding what to do.
> Lets imagine the guest is deciding to use HVC to access the secure firmware
> (AFAIU this patch series is adding that), are you going to monitor all the

Hi Julien,

My patch enables HVC and SMC to reach the platform specific calls. I didn't
connect PSCI over SMC though, I forgot about that but I definitely think
it makes sense to do so. Unmodified guest code would use SMC for everything
when running without a Hypervisor, so preferably the same would work on top
of Xen.

I'll fix that for v3.


> HVCs (including hypercall)?
> Similarly, non-modified baremetal app could use SMC to power on/off the vCPU
> (see PSCI spec). Will you emulate that in the monitor app?
> So I think we need to be consistent across HVC and SMC call. And mutually
> exclusive does not sound right to me for HVC.
> Cheers,
> [1] 
> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0028b/index.html
> -- 
> Julien Grall

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