raw-posix: add raw_get_aio_fd() for virtio-blk-data-plane

The raw_get_aio_fd() function allows virtio-blk-data-plane to get the
file descriptor of a raw image file with Linux AIO enabled.  This
interface is really a layering violation that can be resolved once the
block layer is able to run outside the global mutex - at that point
virtio-blk-data-plane will switch from custom Linux AIO code to using
the block layer.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
configure: add CONFIG_VIRTIO_BLK_DATA_PLANE

The virtio-blk-data-plane feature only works with Linux AIO.  Therefore
add a ./configure option and necessary checks to implement this
dependency.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
dataplane: add host memory mapping code

The data plane thread needs to map guest physical addresses to host
pointers.  Normally this is done with cpu_physical_memory_map() but the
function assumes the global mutex is held.  The data plane thread does
not touch the global mutex and therefore needs a thread-safe memory
mapping mechanism.

Hostmem registers a MemoryListener similar to how vhost collects and
pushes memory region information into the kernel.  There is a
fine-grained lock on the regions list which is held during lookup and
when installing a new regions list.

When the physical memory map changes the MemoryListener callbacks are
invoked.  They build up a new list of memory regions which is finally
installed when the list has been completed.

Note that this approach is not safe across memory hotplug because mapped
pointers may still be in used across memory unplug.  However, this is
currently a problem for QEMU in general and needs to be addressed in the
future.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
dataplane: add virtqueue vring code

The virtio-blk-data-plane cannot access memory using the usual QEMU
functions since it executes outside the global mutex and the memory APIs
are this time are not thread-safe.

This patch introduces a virtqueue module based on the kernel's vhost
vring code.  The trick is that we map guest memory ahead of time and
access it cheaply outside the global mutex.

Once the hardware emulation code can execute outside the global mutex it
will be possible to drop this code.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
dataplane: add event loop

Outside the safety of the global mutex we need to poll on file
descriptors.  I found epoll(2) is a convenient way to do that, although
other options could replace this module in the future (such as an
AioContext-based loop or glib's GMainLoop).

One important feature of this small event loop implementation is that
the loop can be terminated in a thread-safe way.  This allows QEMU to
stop the data plane thread cleanly.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
dataplane: add Linux AIO request queue

The IOQueue has a pool of iocb structs and a function to add new
read/write requests.  Multiple requests can be added before calling the
submit function to actually tell the host kernel to begin I/O.  This
allows callers to batch requests and submit them in one go.

The actual I/O is performed using Linux AIO.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
iov: add iov_discard() to remove data

The iov_discard() function removes data from the front or back of the
vector.  This is useful when peeling off header/footer structs.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
test-iov: add iov_discard() testcase

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
iov: add iov_get_ptr() to reference vector data

The iov_get_ptr() data returns a pointer to contiguous data within a
vector.  This allows the caller to manipulate data inside the vector
without copying in/out using iov_from_buf()/iov_to_buf() when we know
that data is contiguous within an iovec element.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
test-iov: add iov_get_ptr() test case

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
dataplane: add virtio-blk data plane code

virtio-blk-data-plane is a subset implementation of virtio-blk.  It only
handles read, write, and flush requests.  It does this using a dedicated
thread that executes an epoll(2)-based event loop and processes I/O
using Linux AIO.

This approach performs very well but can be used for raw image files
only.  The number of IOPS achieved has been reported to be several times
higher than the existing virtio-blk implementation.

Eventually it should be possible to unify virtio-blk-data-plane with the
main body of QEMU code once the block layer and hardware emulation is
able to run outside the global mutex.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
virtio-blk: add x-data-plane=on|off performance feature

The virtio-blk-data-plane feature is easy to integrate into
hw/virtio-blk.c.  The data plane can be started and stopped similar to
vhost-net.

Users can take advantage of the virtio-blk-data-plane feature using the
new -device virtio-blk-pci,x-data-plane=on property.

The x-data-plane name was chosen because at this stage the feature is
experimental and likely to see changes in the future.

If the VM configuration does not support virtio-blk-data-plane an error
message is printed.  Although we could fall back to regular virtio-blk,
I prefer the explicit approach since it prompts the user to fix their
configuration if they want the performance benefit of
virtio-blk-data-plane.

Limitations:
 * Only format=raw is supported
 * Live migration is not supported
 * Block jobs, hot unplug, and other operations fail with -EBUSY
 * I/O throttling limits are ignored
 * Only Linux hosts are supported due to Linux AIO usage

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>