Re: [PATCH v5 2/5] libxl: add query function for backend support by device model

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

On 30/03/16 14:53, Wei Liu wrote:
> On Wed, Mar 30, 2016 at 02:05:55PM +0200, Juergen Gross wrote:
>> Add a function to query whether the device model is supporting a
>> specific backend type. The device model is writing the supported
>> backend types to Xenstore on startup. The new query function checks
>> for the appropriate entry to be present.
>>
>> As not all versions of qemu are capable to indicate support of
>> specific backends the query function is to be called with an indicator
>> whether the default return value should be "supported" (in case qemu
>> doesn't know set any support information) or "not supported".
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
> 
> The code itself looks straightforward enough.
> 
> But note that this is a new protocol that needs to be supported
> essentially forever. I've CC QEMU maintainers for their input.

FWIW, libxl only needs to support this forever if qemu ever actually
provides it.  If no version of qemu ever ships with this check, then we
can't break anything by removing it.

This is what I was saying in my other comment -- we could just check
this in, and back out the whole series if the qemu side gets NACK'ed.
Even if we release with this, there's no real harm done, as the qusb
code will simply never be activated.

> This also means even if we get this in for 4.7 we can't essentially
> benefit from it until this protocol is implemented in upstream QEMU. At
> this point I don't think I would take this particular patch for 4.7.

qemu has a faster release cycle than us, right?  If we have support in
principle for this approach, people who want qusb support can always use
a newer qemu release.

OTOH, if we check this in then the qemu folks can either take this exact
approach, or something completely incompatible -- they can't make minor
modifications.

 -George

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