Re: [Qemu-devel] [RFC PATCH v1 1/1] vGPU core driver : to provide common interface for vGPU.

[Neo Jia]

On Wed, Feb 17, 2016 at 03:31:24AM +0000, Tian, Kevin wrote:
> > From: Neo Jia [mailto:address@hidden
> > Sent: Tuesday, February 16, 2016 4:49 PM
> > 
> > On Tue, Feb 16, 2016 at 08:10:42AM +0000, Tian, Kevin wrote:
> > > > From: Neo Jia [mailto:address@hidden
> > > > Sent: Tuesday, February 16, 2016 3:53 PM
> > > >
> > > > On Tue, Feb 16, 2016 at 07:40:47AM +0000, Tian, Kevin wrote:
> > > > > > From: Neo Jia [mailto:address@hidden
> > > > > > Sent: Tuesday, February 16, 2016 3:37 PM
> > > > > >
> > > > > > On Tue, Feb 16, 2016 at 07:27:09AM +0000, Tian, Kevin wrote:
> > > > > > > > From: Neo Jia [mailto:address@hidden
> > > > > > > > Sent: Tuesday, February 16, 2016 3:13 PM
> > > > > > > >
> > > > > > > > On Tue, Feb 16, 2016 at 06:49:30AM +0000, Tian, Kevin wrote:
> > > > > > > > > > From: Alex Williamson [mailto:address@hidden
> > > > > > > > > > Sent: Thursday, February 04, 2016 3:33 AM
> > > > > > > > > >
> > > > > > > > > > On Wed, 2016-02-03 at 09:28 +0100, Gerd Hoffmann wrote:
> > > > > > > > > > >   Hi,
> > > > > > > > > > >
> > > > > > > > > > > > Actually I have a long puzzle in this area. Definitely 
> > > > > > > > > > > > libvirt will use
> > UUID
> > > > to
> > > > > > > > > > > > mark a VM. And obviously UUID is not recorded within 
> > > > > > > > > > > > KVM. Then
> > how
> > > > does
> > > > > > > > > > > > libvirt talk to KVM based on UUID? It could be a good 
> > > > > > > > > > > > reference to
> > this
> > > > design.
> > > > > > > > > > >
> > > > > > > > > > > libvirt keeps track which qemu instance belongs to which 
> > > > > > > > > > > vm.
> > > > > > > > > > > qemu also gets started with "-uuid ...", so one can query 
> > > > > > > > > > > qemu via
> > > > > > > > > > > monitor ("info uuid") to figure what the uuid is.  It is 
> > > > > > > > > > > also in the
> > > > > > > > > > > smbios tables so the guest can see it in the system 
> > > > > > > > > > > information table.
> > > > > > > > > > >
> > > > > > > > > > > The uuid is not visible to the kernel though, the kvm 
> > > > > > > > > > > kernel driver
> > > > > > > > > > > doesn't know what the uuid is (and neither does vfio).  
> > > > > > > > > > > qemu uses
> > file
> > > > > > > > > > > handles to talk to both kvm and vfio.  qemu notifies both 
> > > > > > > > > > > kvm and
> > vfio
> > > > > > > > > > > about anything relevant events (guest address space 
> > > > > > > > > > > changes etc)
> > and
> > > > > > > > > > > connects file descriptors (eventfd -> irqfd).
> > > > > > > > > >
> > > > > > > > > > I think the original link to using a VM UUID for the vGPU 
> > > > > > > > > > comes from
> > > > > > > > > > NVIDIA having a userspace component which might get 
> > > > > > > > > > launched from
> > a udev
> > > > > > > > > > event as the vGPU is created or the set of vGPUs within 
> > > > > > > > > > that UUID is
> > > > > > > > > > started.  Using the VM UUID then gives them a way to 
> > > > > > > > > > associate that
> > > > > > > > > > userspace process with a VM instance.  Maybe it could 
> > > > > > > > > > register with
> > > > > > > > > > libvirt for some sort of service provided for the VM, I 
> > > > > > > > > > don't know.
> > > > > > > > >
> > > > > > > > > Intel doesn't have this requirement. It should be enough as 
> > > > > > > > > long as
> > > > > > > > > libvirt maintains which sysfs vgpu node is associated to a VM 
> > > > > > > > > UUID.
> > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > > qemu needs a sysfs node as handle to the vfio device, 
> > > > > > > > > > > something
> > > > > > > > > > > like /sys/devices/virtual/vgpu/<name>.  <name> can be a 
> > > > > > > > > > > uuid if
> > you
> > > > want
> > > > > > > > > > > have it that way, but it could be pretty much anything.  
> > > > > > > > > > > The sysfs node
> > > > > > > > > > > will probably show up as-is in the libvirt xml when 
> > > > > > > > > > > assign a vgpu to
> > a
> > > > > > > > > > > vm.  So the name should be something stable (i.e. when 
> > > > > > > > > > > using a uuid
> > as
> > > > > > > > > > > name you should better not generate a new one on each 
> > > > > > > > > > > boot).
> > > > > > > > > >
> > > > > > > > > > Actually I don't think there's really a persistent naming 
> > > > > > > > > > issue, that's
> > > > > > > > > > probably where we diverge from the SR-IOV model.  SR-IOV 
> > > > > > > > > > cannot
> > > > > > > > > > dynamically add a new VF, it needs to reset the number of 
> > > > > > > > > > VFs to zero,
> > > > > > > > > > then re-allocate all of them up to the new desired count.  
> > > > > > > > > > That has some
> > > > > > > > > > obvious implications.  I think with both vendors here, we 
> > > > > > > > > > can
> > > > > > > > > > dynamically allocate new vGPUs, so I would expect that 
> > > > > > > > > > libvirt would
> > > > > > > > > > create each vGPU instance as it's needed.  None would be 
> > > > > > > > > > created by
> > > > > > > > > > default without user interaction.
> > > > > > > > > >
> > > > > > > > > > Personally I think using a UUID makes sense, but it needs 
> > > > > > > > > > to be
> > > > > > > > > > userspace policy whether that UUID has any implicit meaning 
> > > > > > > > > > like
> > > > > > > > > > matching the VM UUID.  Having an index within a UUID 
> > > > > > > > > > bothers me a
> > bit,
> > > > > > > > > > but it doesn't seem like too much of a concession to enable 
> > > > > > > > > > the use case
> > > > > > > > > > that NVIDIA is trying to achieve.  Thanks,
> > > > > > > > > >
> > > > > > > > >
> > > > > > > > > I would prefer to making UUID an optional parameter, while 
> > > > > > > > > not tieing
> > > > > > > > > sysfs vgpu naming to UUID. This would be more flexible to 
> > > > > > > > > different
> > > > > > > > > scenarios where UUID might not be required.
> > > > > > > >
> > > > > > > > Hi Kevin,
> > > > > > > >
> > > > > > > > Happy Chinese New Year!
> > > > > > > >
> > > > > > > > I think having UUID as the vgpu device name will allow us to 
> > > > > > > > have an gpu
> > vendor
> > > > > > > > agnostic solution for the upper layer software stack such as 
> > > > > > > > QEMU, who is
> > > > > > > > supposed to open the device.
> > > > > > > >
> > > > > > >
> > > > > > > Qemu can use whatever sysfs path provided to open the device, 
> > > > > > > regardless
> > > > > > > of whether there is an UUID within the path...
> > > > > > >
> > > > > >
> > > > > > Hi Kevin,
> > > > > >
> > > > > > Then it will provide even more benefit of using UUID as libvirt can 
> > > > > > be
> > > > > > implemented as gpu vendor agnostic, right? :-)
> > > > > >
> > > > > > The UUID can be VM UUID or vGPU group object UUID which really 
> > > > > > depends on
> > the
> > > > > > high level software stack, again the benefit is gpu vendor agnostic.
> > > > > >
> > > > >
> > > > > There is case where libvirt is not used while another mgmt. stack 
> > > > > doesn't use
> > > > > UUID, e.g. in some Xen scenarios. So it's not about GPU vendor 
> > > > > agnostic. It's
> > > > > about high level mgmt. stack agnostic. That's why we need make UUID as
> > > > > optional in this vGPU-core framework.
> > > >
> > > > Hi Kevin,
> > > >
> > > > As long as you have to create an object to represent vGPU or vGPU 
> > > > group, you
> > > > will have UUID, no matter which management stack you are going to use.
> > > >
> > > > UUID is the most agnostic way to represent an object, I think.
> > > >
> > > > (a bit off topic since we are supposed to focus on VFIO on KVM)
> > > >
> > > > Since now you are talking about Xen, I am very happy to discuss that 
> > > > with you.
> > > > You can check how Xen has managed its object via UUID in xapi.
> > > >
> > >
> > > Well, I'm not the expert in this area. IMHO UUID is just an user level
> > > attribute, which can be associated to any sysfs node and managed by
> > > mgmt. stack itself, and then the sysfs path can be opened as the
> > > bridge between user/kernel. I don't understand the necessity of binding
> > > UUID internally within vGPU core framework here. Alex gave one example
> > > of udev, but I didn't quite catch why only UUID can work there. Maybe
> > > you can elaborate that requirement.
> > 
> > Hi Kevin,
> > 
> > UUID is just a way to represent an object.
> > 
> > It is not binding, it is just a representation. I think here we are just
> > creating a convenient and generic way to represent a virtual gpu device on
> > sysfs.
> > 
> > Having the UUID as part of the virtual gpu device name allows us easily 
> > find out
> > the <vgpu, vgpu_group or VM> mapping.
> > 
> > UUID can be anything, you can always use an UUID to present VMID in the 
> > example
> > you listed below, so you are actually gaining flexibility by using UUID 
> > instead
> > of VMID as it can be supported by both KVM and Xen. :-)
> > 
> > Thanks,
> > Neo
> > 
> 
> Thanks Neo. I understand UUID has its merit in many usages. As you
> may see from my earlier reply, my main concern is whether it's a must
> to record this information within kernel vGPU-core framework. We can
> still make it hypervisor agnostic even not using UUID, as long as there's
> a unified namespace created for all vgpus, like:
>       vgpu-vendor-0, vgpu-vendor-1, ...
> 
> Then high level mgmt. stack can associate UUID to that namespace. So
> I hope you can help elaborate below description:
> 

> > Having the UUID as part of the virtual gpu device name allows us easily 
> > find out
> > the <vgpu, vgpu_group or VM> mapping.
> 

Hi Kevin,

The answer is simple, having a UUID as part of the device name will give you a
unique sysfs path that will be opened by QEMU.

vgpu-vendor-0 and vgpu-vendor-1 will not be unique as we can have multiple
virtual gpu devices per VM coming from same or different physical devices.

If you are worried about losing meaningful name here, we can create a sysfs file
to capture the vendor device description if you like.

Thanks,
Neo

> so we can evaluate carefully whether UUID must be included in the vgpu
> device name or not. If not using UUID is OK, it can remove one more
> user space dependency on using vgpu features.
> 
> Thanks
> Kevin