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

[Julien Grall <julien.grall@xxxxxxx>]

(+ Edgar, Mark, Dave)

Hi,

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.
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.
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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