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

Alex Williamson alex.williamson at redhat.com
Thu Mar 15 14:59:41 UTC 2018


On Thu, 15 Mar 2018 14:33:55 +0000
Daniel P. Berrangé <berrange at redhat.com> wrote:

> 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

Say what?  If a guest is assigned a PF, they get the PF, they don't get
to enable SR-IOV to also get the VFs.  But SR-IOV and multifunction are
far from synonymous nor is SR-IOV ubiquitous to all use cases, so I
don't know why we're bringing SR-IOV into this discussion.

> > > 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.

Since when has this been the case? (nit, the example is domain=0x5,
bus=0x1,...)
 
> 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>

Neither really bothers me, but I'm confused by the claimed existing
handling of SR-IOV.  Either you're assigning a PF and SR-IOV is
irrelevant and unavailable to the guest or you're assigning a VF and,
well, SR-IOV is still mostly irrelevant to libvirt unless someone
decides to assign the PF hosting the VF or libvirt needs to do VF
configuration via the PF.  Thanks,

Alex




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