[libvirt] [PATCH] Device{Attach, Detach}: Document S4 limitations

[Michal Privoznik]

On 30.04.2014 17:46, Eric Blake wrote:
> On 04/30/2014 07:55 AM, Michal Privoznik wrote:
>> https://bugzilla.redhat.com/show_bug.cgi?id=808463
>>
>> Well, libvirt doesn't distinguish between domain poweroff and
>> hibernation (S4). It's hard to differentiate these two on a real
>> machine anyway. As a result, any device that is hot(un-)plugged is
>> lost (appears again) when domain is started again as from our POV
>> it is a fresh cold boot. Instead of doing anything wise here, we
>> should just document this as known limitation.
>
> Umm, doesn't qemu give us a different event when a guest stops due to S4
> than when it stops due to power off?  Yeah, we don't do a good job of
> tracking that a guest is in S4 (since the qemu process no longer
> exists), but that's something that could be improved.

Well, there are some cases where S4 is distinguishable to bare halt. But 
it all depends on the guest kernel playing nicely. I mean, for instance 
take the Linux kernel. If S4 is supported in the ACPI, the it correctly 
sets the S4 in ACPI registers. But if it's not supported in the ACPI, 
then you can still do S4 but you won't be notified about it (can't tell 
out of the box if guest did S4 or halt). And I recall something similar 
with the Windows kernel.

>
>>
>> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
>> ---
>>   src/libvirt.c | 12 ++++++++++++
>>   1 file changed, 12 insertions(+)
>>
>> diff --git a/src/libvirt.c b/src/libvirt.c
>> index b6c99c5..20297eb 100644
>> --- a/src/libvirt.c
>> +++ b/src/libvirt.c
>> @@ -10319,6 +10319,9 @@ virNodeGetSecurityModel(virConnectPtr conn, virSecurityModelPtr secmodel)
>>    * in an existing CDROM/Floppy device, however, applications are
>>    * recommended to use the virDomainUpdateDeviceFlag method instead.
>>    *
>> + * Be aware that a device that is hotplugged may disappear if
>> + * domain resumes from the S4 state (also known as hibernation).
>
> Maybe be a bit more positive:
>
> Be aware that hotplug changes might not persist across a domain going
> into S4 state (also known as hibernation) unless you also modify the
> persistent domain definition.
>

Okay, that sounds better. Do you want me to repost?