Re: [PATCH v8 05/28] virt: gunyah: Add hypercalls to identify Gunyah

[Alex Elder]

On 1/10/23 11:56 AM, Elliot Berman wrote:
> > Is there any need for the endianness of these values to be specified?
> > Does Gunyah operate with a well-defined endianness?  Is there any
> > chance a VM can run with an endianness different from Gunyah?  I
> > see that the arm_smcc_* structures are defined without endianness.
> > (Sorry if these are dumb questions.)
>
> All of the data transfers for hypercalls happen via registers, so 
> endianness doesn't have impact here (there is no "low address" in a 
> register).

I don't believe that is technically true.  Practically speaking,
it's probably almost *always* little-endian.  But for example,
here:

https://developer.arm.com/documentation/102376/0100/Alignment-and-endianness
it says:

    Endianness

    In Armv8-A, instruction fetches are always treated as
    little-endian.

    For data accesses, it is IMPLEMENTATION DEFINED whether
    both little-endian and big-endian are supported. And if
    only one is supported, it is IMPLEMENTATION DEFINED
    which one is supported.

    For processors that support both big-endian and
    little-endian, endianness is configured per Exception
    level.

Perhaps that last sentence doesn't apply to HVC exceptions
but to me it *sounds* like it's at least possible for a VM
to be running with an endianness that differs from the
hypervisor (perhaps not other VMs though.)

This is not an area of expertise of mine, so I would love
for someone who knows more to correct me if I'm wrong.

It's likely to be fine as-is, but (other than the work to
do it and get it right) it doesn't hurt to specify it and
do the conversions as data is passes to/from the hypervisor.

					-Alex