virtio.h: add some user-friendly apis

[CV-Bowen]

• virtio.h: add apis virtio_read/write_config_member

Used to read/write the virtio deivces' configuration space member

• virtio.h: add new api virtio_has_feature()

virtio_has_feature() can be easily used to heck if the virtio device
support a specific feature.

• virtio.h: add new feature bit VIRTIO_F_ANY_LAYOUT

Follow the virtio spec, this feature bit indicates that the device
accepts arbitrary descriptor layouts.

[CV-Bowen]

@arnopo Hi, I meet a CI issue:

#define virtio_read_config_member(vdev, structname, member, _ptr) \
	virtio_read_config((vdev), metal_offset_of(structname, member), \
			   _ptr, sizeof(*(_ptr)))

This code reports MACRO_ARG_REUSE: Macro argument reuse '_ptr' - possible side-effects?
So can we use typeof to avoid this issue?

[arnopo]

This function return valid value only if virtio_negotiate_features has been called first . That also means that it does not work for virtio device role.

[arnopo]

Declaring a local variable in the macro should solve the build issue

#define virtio_write_config_member(vdev, structname, member, _ptr)  do { \
	int ptr = (_ptr); \
	virtio_write_config((vdev), metal_offset_of(structname, member), \
			    ptr, sizeof(*(ptr)));
	} while (0)

[CV-Bowen]

Because I need use the sizeof(member), i find a new way, but have same issue :(.

[arnopo]

Because I need use the sizeof(member), i find a new way, but have same issue :(.

Perhaps this could work:

#define virtio_write_config_member(vdev, structname, member, ptr) do { \
    __typeof__(structname) __tmp_structname = (structname); \
    __typeof__(member) __tmp_member = (member); \
    virtio_write_config(vdev, metal_offset_of(__tmp_structname, __tmp_member), \
                        ptr, sizeof(((__tmp_structname *)0)->__tmp_member)); \
} while (0)

but is it work for every compiler...?

@glneo concerns seems valid. This macro introduces an additional API, but it may not offer significant benefits to the library.

[glneo]

Do you have a user of this new API we could take a look at? Not sure I get the use right now..

[CV-Bowen]

NuttX is a RTOS that we used, and we have implemented many virtio drivers in it, virtio-blk is one for them and use this API:
https://github.com/apache/nuttx/blob/master/drivers/virtio/virtio-blk.c#L584
Now these APIs are placed in NuttX but we think them should be moved to OpenAMP.

[glneo]

Thanks for the info. I'm trying to make sure we do not needlessly grow the OpenAMP API.

virtio_has_feature() seems like a reasonable addition to me as it abstracts access to the contents of the vdev struct (opaque structures with accessors is good design).

'virtio_{read,write}_config_member()' on the other hand looks like a thin wrapper around metal_offset_of(). Accessing structures custom to the transport layer through the virtio layer API feels like a layering issue to begin with, so I want to make sure we really want to keep building more API's on top of that..