Re: [Xen-devel] [PATCH 1/2] arm: smccc: handle SMCs/HVCs according to SMCCC

["Edgar E. Iglesias" <edgar.iglesias@xxxxxxxxxx>]

On Tue, Aug 01, 2017 at 11:59:00AM +0100, Julien Grall wrote:
> (+ Edgar, Mark, Dave)
> 
> Hi,

Hi Julien,

I'll share some thoughts based on our platforms.


> On 14/06/17 15:10, Volodymyr Babchuk wrote:
> >SMCCC (SMC Call Convention) describes how to handle both HVCs and SMCs.
> >SMCCC states that both HVC and SMC are valid conduits to call to a different
> >firmware functions. Thus, for example PSCI calls can be made both by
> >SMC or HVC. Also SMCCC defines function number coding for such calls.
> >Besides functional calls there are query calls, which allows underling
> >OS determine version, UID and number of functions provided by service
> >provider.
> >
> >This patch adds new file `smccc.c`, which handles both generic SMCs
> >and HVC according to SMC. At this moment it implements only one
> >service: Standard Hypervisor Service.
> >
> >Standard Hypervisor Service only supports query calls, so caller can
> >ask about hypervisor UID and determine that it is XEN running.
> >
> >This change allows more generic handling for SMCs and HVCs and it can
> >be easily extended to support new services and functions.
> 
> I have already reviewed the code and one thing I missed is how a domain will
> know that Xen supports SMCCC.
> 
> Currently, Xen will:
>       - inject an undefined instruction for any SMC issued by a guest
>       - crash the guest (quite bad) for any unknown HCV #0
> 
> So a guest needs to be aware whether Xen supports SMCCC convention or not. I
> am not aware of any bindings in the device-tree for doing that.

On our platforms, SW probes the DT for specific service classes and then
probes for specific versions via SMC calls using the standard Version FIDs.
If the DT does not specify the firmware node, I don't think any SMCs will be
issued but the guest may not be functional (as the firmware interface is
mandatory).

I don't know of a generic DT node/compat that tells guests if SMCC probing
is allowed or not. Perhaps there should be one, or there should be yet
another service specific one for Hypervisors. I don't know.

For example, these are the nodes we've got (PSCI and EEMI/SIP):
        psci {
                compatible = "arm,psci-0.2";
                method = "smc";
        };

        pmufw: firmware {
                compatible = "xlnx,zynqmp-pm";
                method = "smc";
                interrupt-parent = <&gic>;
                interrupts = <0 35 4>;
        };

SW that does not have DT support, will either directly probe the SMC
interface or in some cases just assume it's there and use it.

ZynqMP-wise, Xen is in a little bit of an akward position by messing
guests up if they probe for non-existing SMC services but I don't think
it's that bad. Mainly because there's really very little ZynqMP guests
that need the firmware would be able todo without it.

For other platforms and services, I guess FW may very well be optional
and probing makes more sence. So getting support for gracefully returning
Unknown FID still important...


> The other issue is not all the firmware may be SMCCC capable. We may want in
> the future to support other convention to allow baremetal OS running on Xen.
> This means a guest should be able to detect the convention used.

Perhaps this could be done by injecting DT fragments like we do for passthrough?

Cheers,
Edgar

> 
> I don't have a clear answer here yet, but thought it would be good to start
> a conversation.
> 
> Cheers,
> 
> -- 
> Julien Grall

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