Re: [PATCH 2/4] libxl: Add support for blktap vbd3

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Thu, Feb 01, 2024 at 01:30:22PM -0500, Jason Andryuk wrote:
> This patch re-introduces blktap support to libxl.  Unlike earlier
> versions, it does not link against any blktap library.  libxl changes
> are needed to write to the vbd3 backend XenStore nodes.
> 
> blktap has three components.  tapdisk is a daemon implementing the disk
> IO, NBD (Network Block Device), and Xen PV interfaces.  tap-ctl is a
> tool to control tapdisks - creating, starting, stopping and freeing.
> tapback manages the XenStore operations and instructs tapdisk to
> connect.
> 
> It is notable that tapdisk performs the grant and event channel ops, but
> doesn't interact with XenStore.  tapback performs XenStore operations
> and notifies tapdisks of values and changes.
> 
> The flow is: libxl writes to the "vbd3" XenStore nodes and runs the
> block-tap script.  The block-tap script runs tap-ctl to create a tapdisk
> instance as the physical device.  tapback then sees the tapdisk and
> instructs the tapdisk to connect up the PV blkif interface.
> 
> This is expected to work without the kernel blktap driver, so the
> block-tap script is modified accordingly to write the UNIX NBD path.
> 
> backendtype=tap was not fully removed previously, but it would never
> succeed since it would hit the hardcoded error in disk_try_backend().
> It is reused now.
> 
> An example command to attach a vhd:
> xl block-attach vm 
> 'vdev=xvdf,backendtype=tap,format=vhd,target=/srv/target.vhd'
> 
> Format raw also works to run an "aio:" tapdisk.
> 
> Signed-off-by: Jason Andryuk <jandryuk@xxxxxxxxx>

Reviewed-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>

Thanks,

-- 
Anthony PERARD