Re: [Xen-devel] [PATCH 4/6] libxl: Use the block-tap script for LIBXL_DISK_BACKEND_TAP

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 07/07/2015 01:29 PM, Wei Liu wrote:
> On Mon, Jul 06, 2015 at 11:51:41AM +0100, George Dunlap wrote:
>> The block-tap script can now do everything needed for libxl; no need
>> to link against the blktap library.
>>
>> To do this:
>>
>> * Set disk->script to "block-tap" and dev to "format:pdev_path" in
>>   device_disk_add for LIBXL_DISK_BACKEND_TAP
>>
>> * Remove libxl_blktap2.o and libxl_noblktap2.o and all code depending
>>   on them
>>
>> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
>> ---
>> CC: Ian Campbell <ian.campbell@xxxxxxxxxx>
>> CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
>> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
>> CC: Dave Scott <Dave.Scott@xxxxxxxxxxxxx>
>> CC: Jonathan Ludlam <Jonathan.Ludlam@xxxxxxxxxxxxx>
>> CC: Wen Congyang <wency@xxxxxxxxxxxxxx>
>> CC: Yang Hongyang <yanghy@xxxxxxxxxxxxxx>
>> CC: Felipe Franciosi <felipe.franciosi@xxxxxxxxxx>
>>
>> Note: This is broken for HVM domains at the moment because all block
>> scripts are broken for HVM domains.  That is a bug which should be a
>> blocker for the 4.6 release.  As such, I think there is justification
> 
> This bug is partially fixed by this particular patch. I think making
> things partially work is better than having it not work at all.
> 
>> for checking in this "feature" (enabling use of a 'system' blktap)
>> with the assumption that the FIXME will go away before the release.
>>
> 
> I have the impression that to make QEMU work with block script (the
> FIXME) would require us to devise a way to get back the path of block
> device node. Am I right? How much work do you envisage is needed to make
> that FIXME go away?

For block scripts where dom0 is the backend, I *think* we should in
theory be able change block-common to store a pathname to the device
node in xenstore and hand it to qemu.  Alternately, we could fish the
existing major:minor numbers out of xenstore and make our own node to
pass to qemu.

> On the flip side we're not making anything worse than before so we can
> probably get by by writing that down in known issues section of release
> note.

We do technically cause a regression at the moment.  Before this patch,
someone who has access to the blktap kernel module, and is using the
in-tree blktap, will be able to use a blktap backend with HVM guests.
Once this patch is accepted, blktap will work for PV guests, but not for
HVM guests -- at least until we fix the bug with local block scripts.

 -George

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