Linux Virtualization - Jun 2018 - [virtio-dev] Re: [PATCH] qemu: Introduce VIRTIO_NET_F_STANDBY feature bit to virtio_net

On Fri, Jun 22, 2018 at 12:43:26PM -0700, Siwei Liu
wrote:
> > The semantics are that the primary is always used if present in
> > preference to standby.
> OK. If this is the only semantics of what "standby" refers to in
> general, that is fine.
> 
> I just don't want to limit the failover/standby semantics to the
> device model specifics, the "accelerated datapath" thing or
whatever.
> I really don't know where the requirements of the "accelerated
> datapath" came from,
It's a way to put it that matches what failover actually provides.
> as the originial problem is migrating vfio
> devices which is in match of QEMU's live migration model.
If you put it this way then it's natural to require that we have a
config with a working vfio device, and that we somehow add virtio for
duration of migration.
> Hyper-V has
> it's limitation to do 1-netdev should not impact how KVM/QEMU should
> be doing it.
That's a linux thing and pretty orthogonal to host/guest interface.
> > Jason said virtio net is sometimes preferable.
> > If that's the case don't make it a standby.
> >
> > More advanced use-cases do exist and e.g. Alexander Duyck
> > suggested using a switch-dev.
> 
> The switchdev case would need a new feature bit, right?
> 
> -Siwei
I think it would need to be a completely new device.
> > failover isn't it though.
> >
> > --
> > MST

On Fri, Jun 22, 2018 at 2:47 PM, Michael S. Tsirkin <mst at redhat.com>
wrote:
> On Fri, Jun 22, 2018 at 12:43:26PM -0700, Siwei Liu wrote:
>> > The semantics are that the primary is always used if present in
>> > preference to standby.
>> OK. If this is the only semantics of what "standby" refers to
in
>> general, that is fine.
>>
>> I just don't want to limit the failover/standby semantics to the
>> device model specifics, the "accelerated datapath" thing or
whatever.
>> I really don't know where the requirements of the "accelerated
>> datapath" came from,
>
> It's a way to put it that matches what failover actually provides.
>
>> as the originial problem is migrating vfio
>> devices which is in match of QEMU's live migration model.
>
> If you put it this way then it's natural to require that we have a
> config with a working vfio device, and that we somehow add virtio for
> duration of migration.
OK. That's the STANDBY feature sematics as I expect. Not something
like "we have a working virtio, but we need an accelerated datapath
via VFIO when the VM is not migrating". The VFIO is the what needs to
be concerned with, not the virtio. The way I view it, the STANDBY
feature, although present in the virtio device, is served as a
communication channel for the paired VFIO device, and the main purpose
of its feature negotiation process has to be around for migrating the
corresponding VFIO. While it's possible to re-use it as a side way to
achieve "accelerated datapath".

There could be other alternatives for vfio migration though, which do
not need the virtio helper, so don't need to get a STANDBY virtio for
that VFIO device.
>
>> Hyper-V has
>> it's limitation to do 1-netdev should not impact how KVM/QEMU
should
>> be doing it.
>
> That's a linux thing and pretty orthogonal to host/guest interface.
I agree. So don't assume any device model specifics in the host/guest
interface.
>
>> > Jason said virtio net is sometimes preferable.
>> > If that's the case don't make it a standby.
>> >
>> > More advanced use-cases do exist and e.g. Alexander Duyck
>> > suggested using a switch-dev.
>>
>> The switchdev case would need a new feature bit, right?
>>
>> -Siwei
>
> I think it would need to be a completely new device.
So how it relates to virtio or failover?

Regards,
-Siwei
>
>> > failover isn't it though.
>> >
>> > --
>> > MST

On Fri, Jun 22, 2018 at 03:25:19PM -0700, Siwei Liu
wrote:
> On Fri, Jun 22, 2018 at 2:47 PM, Michael S. Tsirkin <mst at
redhat.com> wrote:
> > On Fri, Jun 22, 2018 at 12:43:26PM -0700, Siwei Liu wrote:
> >> > The semantics are that the primary is always used if present
in
> >> > preference to standby.
> >> OK. If this is the only semantics of what "standby"
refers to in
> >> general, that is fine.
> >>
> >> I just don't want to limit the failover/standby semantics to
the
> >> device model specifics, the "accelerated datapath" thing
or whatever.
> >> I really don't know where the requirements of the
"accelerated
> >> datapath" came from,
> >
> > It's a way to put it that matches what failover actually provides.
> >
> >> as the originial problem is migrating vfio
> >> devices which is in match of QEMU's live migration model.
> >
> > If you put it this way then it's natural to require that we have a
> > config with a working vfio device, and that we somehow add virtio for
> > duration of migration.
> 
> OK. That's the STANDBY feature sematics as I expect.
Maybe but that isn't what virtio or hyperv implement.
If someone tells you otherwise, they are mistaken IMHO.
> Not something like "we have a working virtio, but we need an
> accelerated datapath via VFIO when the VM is not migrating". The VFIO
> is the what needs to be concerned with, not the virtio.
> The way I view it, the STANDBY feature, although present in the virtio
> device, is served as a communication channel for the paired VFIO
> device, and the main purpose of its feature negotiation process has to
> be around for migrating the corresponding VFIO. While it's possible to
> re-use it as a side way to achieve "accelerated datapath".
> 
> There could be other alternatives for vfio migration though, which do
> not need the virtio helper, so don't need to get a STANDBY virtio for
> that VFIO device.
> 
> >
> >> Hyper-V has
> >> it's limitation to do 1-netdev should not impact how KVM/QEMU
should
> >> be doing it.
> >
> > That's a linux thing and pretty orthogonal to host/guest
interface.
> 
> I agree. So don't assume any device model specifics in the host/guest
interface.
> 
> >
> >> > Jason said virtio net is sometimes preferable.
> >> > If that's the case don't make it a standby.
> >> >
> >> > More advanced use-cases do exist and e.g. Alexander Duyck
> >> > suggested using a switch-dev.
> >>
> >> The switchdev case would need a new feature bit, right?
> >>
> >> -Siwei
> >
> > I think it would need to be a completely new device.
> 
> So how it relates to virtio or failover?
> 
> Regards,
> -Siwei
It might with time offer a super-set of the features.
> >
> >> > failover isn't it though.
> >> >
> >> > --
> >> > MST