[libvirt] [PATCH v4 0/5] PCI hostdev partial assignment support

[Daniel Henrique Barboza]

On 12/17/19 1:32 PM, Alex Williamson wrote:
> On Tue, 17 Dec 2019 11:25:38 -0500
> Laine Stump <laine at laine.org> wrote:
> 
>> On 12/16/19 6:03 PM, Daniel Henrique Barboza wrote:
>>> About breaking existing configurations, there is the possibility of not
>>> going forward with patch 03, which is enforcing this rule of declaring
>>> all the
>>> IOMMU group. Existing domains will keep working as usual, the option to
>>> unassign devices will still be present, but the user will have to deal
>>> with
>>> the potential QEMU errors if not all PCI devices were detached from
>>> the host.
>>>
>>> In this case, the 'unassigned' type will become more of a ON/OFF
>>> switch to
>>> add/remove the PCI hostdev from the guest without removing it from the
>>> domain XML. It is still useful, but we lose the idea of all the IOMMU
>>> devices being described in the domain XML, which is something Laine
>>> mentioned it would be desirable in one of the RFCs.
>>
>>
>> I don't actually recall saying that :-). I haven't looked in the list
>> archives, but what I *can* imagine myself saying is that only devices
>> mentioned in the XML should be manipulated in any way by libvirt. So,
> 
> +1
>   
>> for example, you shouldn't unbind device X from its host driver if there
>> is nothing in the XML telling you to do that. But if a device isn't
>> mentioned in the XML, and is already bound to some driver that is
>> acceptable to the VFIO subsystem (e.g. vfio-pci, pci-stub or no driver
>> at all (? is that right Alex?)) then that should not create any problem.
> 
> Yes, that's right.
> 
>> Doing otherwise would break too many existing configs. (For example, my
>> own assigned-GPU config, which assumes that all the devices are already
>> bound to the proper driver, and uses "managed='no'")


This is the new approach of the series I implemented today and plan to
to send for review today/tomorrow. I realized, after all the discussions
yesterday with Alex, that the patch series would be best just sticking with
what we want fixed (managed=yes and parcial assignment) and leaving
unmanaged (managed=no) configurations alone. If the user has an existing,
working unmanaged setup, this means that the user chose to manage device
detach/re-attach manually and shouldn't be bothered with a change that's
aimed to managed devices.


Thanks,


DHB

> 
> Effectively anyone assigning PFs with a PCIe root port that does not
> support ACS would be broken by this series.  That's a significant
> portion of vfio users.  Thanks,
> 
> Alex
>