Re: [libvirt] [PATCH v4 0/5] PCI hostdev partial assignment support
On Mon, 16 Dec 2019 20:24:56 -0300
Daniel Henrique Barboza <danielhb413(a)gmail.com> wrote:
On 12/16/19 8:06 PM, Alex Williamson wrote:
> On Mon, 16 Dec 2019 17:28:28 -0500
> Cole Robinson <crobinso(a)redhat.com> wrote:
>
>> On 12/16/19 8:36 AM, Daniel Henrique Barboza wrote:
>>> changes from version 3 [1]:
>
> Thanks for catching this! PCIe root ports or bridges being part of an
> IOMMU group is part of the nature of the grouping. However, only
> endpoint devices can be bound to vfio-pci and thus participate in this
> "partial assignment". If the code is trying to force all other devices
> in the IOMMU group that aren't assigned into this partial assignment
> mode, that's just fundamentally broken. Thanks,
The code isn't forcing a device to be assigned to the guest. It is forcing
all the IOMMU devices to be declared in the domain XML to be detached from
the host.
Detached from the host by unbinding from host drivers and binding to
vfio-pci and "partially" assigned to the guest? That's wrong, we can't
do that. Not only will vfio-pci not bind to anything but endpoints,
you'll break the host binding bridges that might be part of the group,
and there are valid use cases for sequestering a device with pci-stub
rather than vfio-pci to add another barrier to the user getting access
to the device.
What I did was to extend a verification Libvirt already does, to
check for
PCI devices of the same IOMMU X being used by other domains, to check the
the host as well. Guest start fails if there is any device left in IOMMU X
that's not present in the domain.
Yep, can't do that.
In short, the code is implying that all IOMMU devices must be
detached from
the host, regardless of whether they're going to be used in the guest,
regardless of whether they are PCI root ports or bridges. Is this assumption
correct, considering kernel/QEMU?
Nope, please don't do this. Thanks,
Alex