Re: [PATCH RFC 2/4] build: Download and build extnernal blktap2.5 repo

[George Dunlap <george.dunlap@eu.citrix.com>]

On 03/30/2015 03:33 PM, Wei Liu wrote:
> On Thu, Mar 26, 2015 at 12:46:08PM +0000, George Dunlap wrote:
>> Download and build XenServer's blktap as an external tree, similar to
>> qemu-xen.
>>
>> As of this patch we just download and build it, but don't install or
>> use it.
>>
>> Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
>> ---
>>
>> FIXME: Directly use the XenServer github repo for now, while we're
>> discussing things.  If we decide to take this series, we'll have to
>> clone the tree on xenbits and remove the FIXME line.
>>
> 
> IMHO we need to support --with-system-blktap= in configure in case
> distro wants to package blktap separately. Not sure if in practice this
> makes sense since AIUI blktap is only used by Xen.

I agree that would be ideal; however, it's not so simple, because at the
moment libxl links directly against libblktap.  This would mean:

1) Changing libxl so that it could dynamically detect the presence of
the proper version of libblktap at runtime and use the stubbed-out
defaults if not available.

2) Changing libxl to exec() tapctl rather than making library calls.
(This might involve making sure that tapctl actually has all the
functionality we need.)

#1 is even more difficult because at the moment blktap isn't really
designed to have stable external versions -- it would involve adding a
library version and all the fun stuff we already do with libxl.

#2 has another potential advantage: using the command-line may make it
easier to be forward-compatible, since adding options won't break
(unlike adding arguments to a function signature).

(Although, I think for API compatibility, changing the public functions
to use structs is probably a more reliable thing to do.)

Either way, I think that's something we should start to look at after
pivoting to the external blktap repo.

 -George