[libvirt] [PATCH RFC] libxl: set serial source path for console type=serial

[Jim Fehlig]

On 06/22/2016 06:07 AM, Cole Robinson wrote:
> On 06/21/2016 10:32 PM, Jim Fehlig wrote:
>> On 06/21/2016 04:20 AM, Joao Martins wrote:
>>> On 06/21/2016 01:38 AM, Jim Fehlig wrote:
>>>> Joao Martins wrote:
>>>>> Guests use a <console /> (and sometimes <serial /> pair) to represent
>>>>> the console. On the callback that is called when console is brought up
>>>>> (NB: before domain starts), we fill the path of the console element with
>>>>> the appropriate "/dev/pts/X". For PV guests it all works fine, although
>>>>> for HVM guests it doesn't. Consequently we end up seeing erronous
>>>>> behaviour on HVM guests where console path is not displayed on the XML
>>>>> but still can be accessed with virDomainOpenConsole after booting guest.
>>>>> Consumers of this XML (e.g. Openstack) also won't be able to use the
>>>>> console path.
>>>> Can you provide example input domXML config that causes this problem?
>>>>
>>> Ah yes - I probably should include that in the commit description?
>> Yes, I think it helps clarify the problem  being solved by the commit.
>>
>>> So, for PV guests for an input console XML element:
>>>
>>>     <console type='pty'>
>>>       <target type='xen' port='0'/>
>>>     </console>
>>>
>>> Which then on domain start gets filled like:
>>>
>>>     <console type='pty' tty='/dev/pts/3'>
>>>       <source path='/dev/pts/3'/>
>>>       <target type='xen' port='0'/>
>>>     </console>
>>>
>>> Although for HVM guests we have:
>>>
>>>     <serial type='pty'>
>>>       <target port='0'/>
>>>     </serial>
>>>     <console type='pty'>
>>>       <target type='serial' port='0'/>
>>>     </console>
>>>
>>> And no changes happen i.e. source path isn't written there _even though_ it's
>>> set on the console element - being the reason why we can access the console in the
>>> first place. IIUC The expected output should be:
>>>
>>>     <serial type='pty'>
>>>       <source path='/dev/pts/4'/>
>>>       <target port='0'/>
>>>     </serial>
>>>     <console type='pty' tty='/dev/pts/4'>
>>>       <source path='/dev/pts/4'/>
>>>       <target type='serial' port='0'/>
>>>     </console>
>> I asked for an example after having problems testing your patch, but it turned
>> out to be an issue which I think was introduced long ago by commit 482e5f15. I
>> was testing with transient domains and noticed 'virsh create; 'virsh console'
>> always failed with
>>
>> error: internal error: character device <null> is not using a PTY
>>
>> My config only contained
>>
>>   <console type='pty'>
>>     <target port='0'/>
>>   </console>
>>
>> so the "really crazy backcompat stuff for consoles" in
>> virDomainDefAddConsoleCompat() moved the config over to def->serials[0] and
>> created a new def->consoles[0] without def->consoles[0].source.type set to
>> VIR_DOMAIN_CHR_TYPE_PTY. Without source.type set, libxlConsoleCallback() would
>> ignore the console and not copy the PTY path to source.data.file.path. 'virsh
>> console' would then fail with the error noted above.
>>
>> I spent too much time debugging this, partly because I was confused by odd
>> behavior with persistent domains. E.g. 'virsh define; virsh start; virsh
>> console' would fail with the same error, but all subsequent 'virsh shutdown;
>> virsh start; virsh console' sequences would work :-). That behavior was due to
>> vm->newDef being populated with correct console config when calling
>> virDomainObjSetDefTransient() in libxlDomainStart(). After the first shutdown of
>> the domain, vm->newDef would be copied over to vm->def, allowing subsequent
>> 'virsh start; virsh console' to work correctly.
>>
> Hmm, that sounds weird. Can you explain more how exactly the XML changes?

It only changes when defining the vm.

>  What
> is the diff between the initial 'virsh define; virsh dumpxml'

Initial XML has

  <console type='pty'>
    <target port='0'/>
  </console>
 
After define

  <serial type='pty'>
    <target port='0'/>
  </serial>
  <console type='pty'>
    <target type='serial' port='0'/>
  </console>

The config doesn't change after start; shutdown. But the contents of
virDomainDef does change, specifically def->consoles[0]->source.type changes
from VIR_DOMAIN_CHR_TYPE_PTY to VIR_DOMAIN_CHR_TYPE_NULL. And as mentioned
below, libxlConsoleCallback() only checks for source.type ==
VIR_DOMAIN_CHR_TYPE_PTY before copying the PTY path to
def->consoles[0]->source.data.file.path. Another potential fix I also mentioned
below is to loosen that condition in libxlConsoleCallback(). I.e. copy the path
if source.type equals VIR_DOMAIN_CHR_TYPE_PTY or VIR_DOMAIN_CHR_TYPE_NULL (and
perhaps VIR_DOMAIN_CHR_TYPE_FILE too?).

Regards,
Jim

>  and the dumpxml
> after the VM has started+shutdown once? Whatever that diff is, and why it's
> not in the XML to begin with, sounds like the root issue to me.
>
>> Long story short, I found the problem but not sure how to fix it :-). One
>> approach would be the below patch. Another would be to loosen the restriction in
>> libxlConsoleCallback, filing in source.data.file.path when the console source
>> type is also set to VIR_DOMAIN_CHR_TYPE_NULL. Adding Cole and Peter (both of
>> which have toiled in and likely loathe this code) to see if they have opinions
>> on a proper fix.
>>
>> Regards,
>> Jim
>>
>>
>> From a231636e8eec08152220e32af5f4026a50954f41 Mon Sep 17 00:00:00 2001
>> From: Jim Fehlig <jfehlig at suse.com>
>> Date: Tue, 21 Jun 2016 20:25:23 -0600
>> Subject: [PATCH] domain conf: copy source type from stolen console
>>
>> When domXML contains only <console type='pty'> and no corresponding
>> <serial>, the console is "stolen" [1] and used as the first <serial>
>> device. A new console is created as an alias to the first <serial>
>> device, but missed copying some configuration such as the source
>> 'type' attribute.
>>
>> [1] See comments associated with virDomainDefAddConsoleCompat() in
>>     $LIBVIRT-SRC/src/conf/domain_conf.c:
>>
>> Signed-off-by: Jim Fehlig <jfehlig at suse.com>
>> ---
>>  src/conf/domain_conf.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
>> index 75ad03f..2fda4fe 100644
>> --- a/src/conf/domain_conf.c
>> +++ b/src/conf/domain_conf.c
>> @@ -3845,6 +3845,7 @@ virDomainDefAddConsoleCompat(virDomainDefPtr def)
>>              /* Create an console alias for the serial port */
>>              def->consoles[0]->deviceType = VIR_DOMAIN_CHR_DEVICE_TYPE_CONSOLE;
>>              def->consoles[0]->targetType =
>> VIR_DOMAIN_CHR_CONSOLE_TARGET_TYPE_SERIAL;
>> +            def->consoles[0]->source.type = def->serials[0]->source.type;
>>          }
>>      } else if (def->os.type == VIR_DOMAIN_OSTYPE_HVM && def->nserials > 0 &&
>>                 def->serials[0]->deviceType == VIR_DOMAIN_CHR_DEVICE_TYPE_SERIAL &&
>>
> Then again this also seems okay to me, but it's concerning that I applied this
> but it didn't cause the test suite to change at all.
>
> - Cole
>