[libvirt] [RFC PATCH 00/28] Enable multifunction pci hotplug

[Daniel P. Berrangé]

On Thu, Mar 15, 2018 at 07:54:47PM +0530, Shivaprasad G Bhat wrote:
> 
> 
> On 03/15/2018 03:31 PM, Daniel P. Berrangé wrote:
> > On Wed, Mar 14, 2018 at 10:44:30PM +0530, Shivaprasad G Bhat wrote:
> > > Hi All,
> > > 
> > > I have revisited/rewritten my previously posted patches. Here is
> > > the RFC. Since this patchset is a complete rewrite, I am starting
> > > with v1 here.
> > > 
> > > The semantics is as discussed before
> > > https://www.redhat.com/archives/libvir-list/2016-April/msg01057.html
> > > 
> > > As I went on to refactor the code to support multifunction virtio devices,
> > > I realised the abort/cleanup path would be a nightmare there, in case of
> > > failures. So, dropped that attempt. The current RFC limits to the real
> > > practical use cases of Multifunction PCI hostdevices. All new test code
> > > to support multifunction PCI hostdevices and test cases are added to
> > > prove the functionality.
> > I guess I'm not really understanding the use case here.  With SRIOV
> > devices, you can already choose between assigning either the physical
> > function (which gives the guest access to all virtual functions), or
> > to assign an arbitrary set of individiual functions to various guests.
> > Why do we need to be able to list many <hostdev> at the same time
> > when hotplugging to assign multiple functions.
> > 
> > Basically can you provide a full description of the problem you are
> > trying to solve and why existing functionality isn't sufficient.
> 
> Hi Daniel,
> 
> This is for cards which may not necessarily be networking cards. Or may be a
> mix of
> networking and storage.
> 
> Suppose, user has below card
> 0005:01:00.0 Ethernet controller: Emulex Corporation OneConnect NIC (Lancer)
> (rev 10)
> 0005:01:00.1 Ethernet controller: Emulex Corporation OneConnect NIC (Lancer)
> (rev 10)
> 0005:01:00.2 Ethernet controller: Emulex Corporation OneConnect NIC (Lancer)
> (rev 10)
> 0005:01:00.3 Ethernet controller: Emulex Corporation OneConnect NIC (Lancer)
> (rev 10)
> 0005:01:00.4 Fibre Channel: Emulex Corporation OneConnect FCoE Initiator
> (Lancer) (rev 10)
> 0005:01:00.5 Fibre Channel: Emulex Corporation OneConnect FCoE Initiator
> (Lancer) (rev 10)

Ok, so this is a device with many functions, but which isn't SRIOV
based, and the goal is to assign the physical device to the guest,
such that guest has all functions available.

> If user wants to hotplug this card to guest, He has to detach all the
> functions from host driver,
> then hotplug 0005:01:00.0, 0005:01:00.1, so on individually. But, today with
> each hotplug
> of the function, each <hostdev> goes to different guest slot. Whereas, PCI
> requires all of
> them to be on the same slot. This is not supported on libvirt today.
> 
> The multifunction cards cant be hotplugged to guest today with the
> individual
> <hostdev>, as the operation is queued by qemu till the function zero of
> guest slot is
> hotplugged. On function zero hotplug, the qemu sends out the event to guest
> for device probing where all the previously hotplugged functions from the
> same slot are discovered. So, grouping the <hostdev>s within the <devices>
> would become necessary to make the whole thing a single operation.

So IIUC, from the patches, if the user wants to assign the physical
device to the guest, they would need to provide XML that looked like
this to the virDomainAttachDevice() method:

    <devices>
      <hostdev mode='subsystem' type='pci' managed='yes'>
        <driver name='vfio'/>
        <source>
          <address domain='0x0000' bus='0x05' slot='0x1' function='0x0'/>
        </source>
      </hostdev>
      <hostdev mode='subsystem' type='pci' managed='yes'>
        <driver name='vfio'/>
        <source>
          <address domain='0x0000' bus='0x05' slot='0x1' function='0x1'/>
        </source>
      </hostdev>
      <hostdev mode='subsystem' type='pci' managed='yes'>
        <driver name='vfio'/>
        <source>
          <address domain='0x0000' bus='0x05' slot='0x1' function='0x2'/>
        </source>
      </hostdev>
      <hostdev mode='subsystem' type='pci' managed='yes'>
        <driver name='vfio'/>
        <source>
          <address domain='0x0000' bus='0x05' slot='0x1' function='0x3'/>
        </source>
      </hostdev>
      <hostdev mode='subsystem' type='pci' managed='yes'>
        <driver name='vfio'/>
        <source>
          <address domain='0x0000' bus='0x05' slot='0x1' function='0x4'/>
        </source>
      </hostdev>
      <hostdev mode='subsystem' type='pci' managed='yes'>
        <driver name='vfio'/>
        <source>
          <address domain='0x0000' bus='0x05' slot='0x1' function='0x5'/>
        </source>
      </hostdev>
    </devices>


Where as if the device were SRIOV based, they would only have to
provide

    <device>
      <hostdev mode='subsystem' type='pci' managed='yes'>
        <driver name='vfio'/>
        <source>
          <address domain='0x0000' bus='0x05' slot='0x1' function='0x0'/>
        </source>
      </hostdev>
    </device>

for the guest to get access to all functions.

I find this difference in behaviour and approach really unpleasant.

I think that they user should only need to provide the the address
of the physical device, in both cases. At most perhaps we need a
new attribute  multifunction="on" on the source address to tell
libvirt that it should attach all the functions, not just the
first

    <device>
      <hostdev mode='subsystem' type='pci' managed='yes'>
        <driver name='vfio'/>
        <source>
          <address domain='0x0000' bus='0x05' slot='0x1' function='0x0' mutlifunction="on"/>
        </source>
      </hostdev>
    </device>


Regards,
Daniel
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