vdpa: introduce dedicated descriptor group for virtqueue

vdpa: introduce dedicated descriptor group for virtqueue

In some cases, the access to the virtqueue's descriptor area, device
and driver areas (precluding indirect descriptor table in guest memory)
may have to be confined to a different address space than where its
buffers reside. Without loss of simplicity and generality with already
established terminology, let's fold up these 3 areas and call them
as a whole as descriptor table group, or descriptor group for short.
Specifically, in case of split virtqueues, descriptor group consists of
regions for Descriptor Table, Available Ring and Used Ring; for packed
virtqueues layout, descriptor group contains Descriptor Ring, Driver
and Device Event Suppression structures.

The group ID for a dedicated descriptor group can be obtained through a
new .get_vq_desc_group() op. If driver implements this op, it means that
the descriptor, device and driver areas of the virtqueue may reside
in a dedicated group than where its buffers reside, a.k.a the default
virtqueue group through the .get_vq_group() op.

In principle, the descriptor group may or may not have same group ID
as the default group. Even if the descriptor group has a different ID,
meaning the vq's descriptor group areas can optionally move to a
separate address space than where guest memory resides, the descriptor
group may still start from a default address space, same as where its
buffers reside. To move the descriptor group to a different address
space, .set_group_asid() has to be called to change the ASID binding
for the group, which is no different than what needs to be done on any
other virtqueue group. On the other hand, the .reset() semantics also
applies on descriptor table group, meaning the device reset will clear
all ASID bindings and move all virtqueue groups including descriptor
group back to the default address space, i.e. in ASID 0.

QEMU's shadow virtqueue is going to utilize dedicated descriptor group
to speed up map and unmap operations, yielding tremendous downtime
reduction by avoiding the full and slow remap cycle in SVQ switching.

Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20231018171456.1624030-4-dtatulea@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Tested-by: Si-Wei Liu <si-wei.liu@oracle.com>
Tested-by: Lei Yang <leiyang@redhat.com>