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Re: [libvirt] [PATCH 1/3] qemu: Allow to specify the sysfs root for qemuBuildCommandLine

On 05/15/2013 07:23 AM, Osier Yang wrote:
> On 15/05/13 19:09, Daniel P. Berrange wrote:
>> On Wed, May 15, 2013 at 12:07:42PM +0100, Daniel P. Berrange wrote:
>>> On Wed, May 15, 2013 at 06:52:44PM +0800, Osier Yang wrote:
>>>> Since 0d70656afded, it starts to access the sysfs files to build
>>>> the qemu command line (by virSCSIDeviceGetSgName, which is find
>>>> out the scsi generic device name by adpater:bus:target:unit), there
>>>> is no way to work around, qemu wants to see the scsi generic device
>>>> like "/dev/sg6" anyway.
>>>> And I think it's not only the place which needs to access sysfs
>>>> files when building qemu command line in future.
>>>> As a solution, this introduces a new argument "sysfs_root" for
>>>> qemuBuildCommandLine, and thus the tests can feed the fake sysfs
>>>> root to it.
>>> IMHO it would be nicer to detach the command line generator
>>> code from any use of sysfs in this case.
>>> Instead of having QEMU call virSCSIDeviceGetSgName() directly,
>>> pass in a callback for it to use to resolve the device path.
>>> eg
>>>    typedef char *(*qemuGetSCSIDevice)(const char *adapter,
>>>                                       unsigned int bus,
>>>                                       unsigned int target,
>>>                                       unsigned int unit);
> Nice idea.
>>> the qemuProcessStart code can pass in an impl of that callback
> I guess you meant qemuBuildCommandLine or so, tests won't
> call  qemuProcessStart. to start the guest.

But qemuProcessStart() calls qemuBuildCommandLine().
>>> which calls  virSCSIDeviceGetSgName(), while the test suite
>>> can pass in an impl which returns a hard-coded device string.
>> In fact there are probably other cases where we should pass in
>> callbacks like this to the QEMU command line builder.

In that same category, the calls to qemuNetworkIfaceConnect,
qemuPhysIfaceConnect, and qemuOpenVhostNet (and the "vmop" param to
qemuBuildCommandLine()) have been bothering me for awhile, but instead
of a callback sent to qemuBuildCommandLine(), I was considering the more
direct idea of opening all the sockets before even calling
qemuBuildCommandLine() and sending them in the arglist "somehow"[*]
(rather than sending in a pointer to a callback that opens the fd's and
passes them back, which gets a bit confusing if you're not intimately
familiar with the code).

[*] I *think* a reasonable way to do this would be to add alist of tap
fd's and a hidden vhost-net fd) to the virActualNetDef that is part of
every virNetDef, then populate it as a part of
networkAllocateActualDevice() (which could probably do with a name
change). This would have the side effect of consolidating all network
device setup into the network driver, thus making it simpler to split it
off into its own service.

>> Instead of
>> adding many parameters to the API, we could provide a struct
> Agreed, many params is always not good. .
>> containing all the various callbacks needed in one go.
>> Daniel

Maybe each device object could have a "hidden" part (i.e. not
necessarily visible in the XML) that can be setup before calling
qemuBuildCommandLine(), and qemuBuildCommandline could use that info if
it's there, otherwise fills in "null" args on the commandline.

I'm not against using a callback when it's appropriate, but I think they
hinder proper understanding of code by newcomers, and should be used
sparingly unless necessary.

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