[libvirt] [PATCH 5/6] qemu: Reduce memlock limit after detaching hostdev

[John Ferlan]

On 12/10/2015 10:03 AM, Andrea Bolognani wrote:
> On Wed, 2015-12-09 at 15:15 -0500, John Ferlan wrote:
>> On 11/24/2015 08:56 AM, Andrea Bolognani wrote:
>>> We increase the limit before plugging in a PCI hostdev or a memory
>>> module because some memory might need to be locked due to eg. VFIO.
>>>
>>> Of course we should do the opposite after unplugging a device: this
>>> was already the case for memory modules, but not for hostdevs.
>>> ---
>>>   src/qemu/qemu_hotplug.c | 6 ++++++
>>>   1 file changed, 6 insertions(+)
>>>
>>> diff --git a/src/qemu/qemu_hotplug.c b/src/qemu/qemu_hotplug.c
>>> index a5c134a..04baeff 100644
>>> --- a/src/qemu/qemu_hotplug.c
>>> +++ b/src/qemu/qemu_hotplug.c
>>> @@ -3070,6 +3070,12 @@ qemuDomainRemoveHostDevice(virQEMUDriverPtr driver,
>>>           networkReleaseActualDevice(vm->def, net);
>>>           virDomainNetDefFree(net);
>>>       }
>>> +
>>> +    /* QEMU might no longer need to lock as much memory, eg. we just detached
>>> +     * a VFIO device, so adjust the limit here */
>>> +    if (qemuDomainAdjustMaxMemLock(vm) < 0)
>>> +        VIR_WARN("Failed to adjust locked memory limit");
>>> +
>>>       ret = 0;
>>>   
>>>    cleanup:
>>>
>>
>> Since the add was in qemuDomainAttachHostPCIDevice, why is the remove
>> not in qemuDomainDetachHostPCIDevice?  Doing it here would mean it's
>> call for any host device removal - whether or not it was ever added.
> 
> Because qemuDomainDetachHostPCIDevice() is where we ask for the device
> to be removed from the guest, but we have to wait for it to actually
> be removed (and for the guest definition to be updated) before changing
> the memory locking limit.
> 
> I guess I could move this code to qemuDomainDetachThisHostDevice(), but
> keeping it close to where the guest definition is updated looks cleaner
> to me.
> 

Yeah - this all got confusing as I was paging back and forth...
"DetachDevice" and "RemoveDevice"... Then I reviewed another change for
shmem and got even more confused.

> I also fail to see how a device that was never added could be removed,
> could you please elaborate?

Since qemuDomainRemoveHostDevice could be called for any host device
removal and not specifically the HostPCIDevice, I was pointing out by
calling qemuDomainAdjustMaxMemLock there you could be calling it even
when a HostPCIDevice wasn't being removed.

It was clear to me when I wrote it ;-)

John