Linux Virtualization - Feb 2018 - [RFC PATCH v3 0/3] Enable virtio_net to act as a backup for a passthru device

On 2/21/2018 5:59 PM, Siwei Liu wrote:
> On Wed, Feb 21, 2018 at 4:17 PM, Alexander Duyck
> <alexander.duyck at gmail.com> wrote:
>> On Wed, Feb 21, 2018 at 3:50 PM, Siwei Liu <loseweigh at
gmail.com> wrote:
>>> I haven't checked emails for days and did not realize the new
revision
>>> had already came out. And thank you for the effort, this revision
>>> really looks to be a step forward towards our use case and is close
to
>>> what we wanted to do. A few questions in line.
>>>
>>> On Sat, Feb 17, 2018 at 9:12 AM, Alexander Duyck
>>> <alexander.duyck at gmail.com> wrote:
>>>> On Fri, Feb 16, 2018 at 6:38 PM, Jakub Kicinski <kubakici at
wp.pl> wrote:
>>>>> On Fri, 16 Feb 2018 10:11:19 -0800, Sridhar Samudrala
wrote:
>>>>>> Ppatch 2 is in response to the community request for a
3 netdev
>>>>>> solution.  However, it creates some issues we'll
get into in a moment.
>>>>>> It extends virtio_net to use alternate datapath when
available and
>>>>>> registered. When BACKUP feature is enabled, virtio_net
driver creates
>>>>>> an additional 'bypass' netdev that acts as a
master device and controls
>>>>>> 2 slave devices.  The original virtio_net netdev is
registered as
>>>>>> 'backup' netdev and a passthru/vf device with
the same MAC gets
>>>>>> registered as 'active' netdev. Both
'bypass' and 'backup' netdevs are
>>>>>> associated with the same 'pci' device.  The
user accesses the network
>>>>>> interface via 'bypass' netdev. The
'bypass' netdev chooses 'active' netdev
>>>>>> as default for transmits when it is available with link
up and running.
>>>>> Thank you do doing this.
>>>>>
>>>>>> We noticed a couple of issues with this approach during
testing.
>>>>>> - As both 'bypass' and 'backup' netdevs
are associated with the same
>>>>>>    virtio pci device, udev tries to rename both of them
with the same name
>>>>>>    and the 2nd rename will fail. This would be OK as
long as the first netdev
>>>>>>    to be renamed is the 'bypass' netdev, but
the order in which udev gets
>>>>>>    to rename the 2 netdevs is not reliable.
>>>>> Out of curiosity - why do you link the master netdev to the
virtio
>>>>> struct device?
>>>> The basic idea of all this is that we wanted this to work with
an
>>>> existing VM image that was using virtio. As such we were trying
to
>>>> make it so that the bypass interface takes the place of the
original
>>>> virtio and get udev to rename the bypass to what the original
>>>> virtio_net was.
>>> Could it made it also possible to take over the config from VF
instead
>>> of virtio on an existing VM image? And get udev rename the bypass
>>> netdev to what the original VF was. I don't say tightly binding
the
>>> bypass master to only virtio or VF, but I think we should provide
both
>>> options to support different upgrade paths. Possibly we could tweak
>>> the device tree layout to reuse the same PCI slot for the master
>>> bypass netdev, such that udev would not get confused when renaming
the
>>> device. The VF needs to use a different function slot afterwards.
>>> Perhaps we might need to a special multiseat like QEMU device for
that
>>> purpose?
>>>
>>> Our case we'll upgrade the config from VF to virtio-bypass
directly.
>> So if I am understanding what you are saying you are wanting to flip
>> the backup interface from the virtio to a VF. The problem is that
>> becomes a bit of a vendor lock-in solution since it would rely on a
>> specific VF driver. I would agree with Jiri that we don't want to
go
>> down that path. We don't want every VF out there firing up its own
>> separate bond. Ideally you want the hypervisor to be able to manage
>> all of this which is why it makes sense to have virtio manage this and
>> why this is associated with the virtio_net interface.
> No, that's not what I was talking about of course. I thought you
> mentioned the upgrade scenario this patch would like to address is to
> use the bypass interface "to take the place of the original virtio,
> and get udev to rename the bypass to what the original virtio_net
> was". That is one of the possible upgrade paths for sure. However the
> upgrade path I was seeking is to use the bypass interface to take the
> place of original VF interface while retaining the name and network
> configs, which generally can be done simply with kernel upgrade. It
> would become limiting as this patch makes the bypass interface share
> the same virtio pci device with virito backup. Can this bypass
> interface be made general to take place of any pci device other than
> virtio-net? This will be more helpful as the cloud users who has
> existing setup on VF interface don't have to recreate it on virtio-net
> and VF separately again.
Yes. This sounds interesting. Looks like you want an existing VM image with
VF only configuration to get transparent live migration support by adding
virtio_net with BACKUP feature.? We may need another feature bit to switch
between these 2 options.

>
>> The other bits get into more complexity then we are ready to handle
>> for now. I think I might have talked about something similar that I
>> was referring to as a "virtio-bond" where you would have a
PCI/PCIe
>> tree topology that makes this easier to sort out, and the
"virtio-bond
>> would be used to handle coordination/configuration of a much more
>> complex interface.
> That was one way to solve this problem but I'd like to see simple ways
> to sort it out.
>
>>>>> FWIW two solutions that immediately come to mind is to
export "backup"
>>>>> as phys_port_name of the backup virtio link and/or assign a
name to the
>>>>> master like you are doing already.  I think team uses
team%d and bond
>>>>> uses bond%d, soft naming of master devices seems quite
natural in this
>>>>> case.
>>>> I figured I had overlooked something like that.. Thanks for
pointing
>>>> this out. Okay so I think the phys_port_name approach might
resolve
>>>> the original issue. If I am reading things correctly what we
end up
>>>> with is the master showing up as "ens1" for example
and the backup
>>>> showing up as "ens1nbackup". Am I understanding that
right?
>>>>
>>>> The problem with the team/bond%d approach is that it creates a
new
>>>> netdevice and so it would require guest configuration changes.
>>>>
>>>>> IMHO phys_port_name == "backup" if BACKUP bit is
set on slave virtio
>>>>> link is quite neat.
>>>> I agree. For non-"backup" virio_net devices would it
be okay for us to
>>>> just return -EOPNOTSUPP? I assume it would be and that way the
legacy
>>>> behavior could be maintained although the function still
exists.
>>>>
>>>>>> - When the 'active' netdev is unplugged OR not
present on a destination
>>>>>>    system after live migration, the user will see 2
virtio_net netdevs.
>>>>> That's necessary and expected, all configuration
applies to the master
>>>>> so master must exist.
>>>> With the naming issue resolved this is the only item left
outstanding.
>>>> This becomes a matter of form vs function.
>>>>
>>>> The main complaint about the "3 netdev" solution is a
bit confusing to
>>>> have the 2 netdevs present if the VF isn't there. The idea
is that
>>>> having the extra "master" netdev there if there
isn't really a bond is
>>>> a bit ugly.
>>> Is it this uglier in terms of user experience rather than
>>> functionality? I don't want it dynamically changed between
2-netdev
>>> and 3-netdev depending on the presence of VF. That gets back to my
>>> original question and suggestion earlier: why not just hide the
lower
>>> netdevs from udev renaming and such? Which important observability
>>> benefits users may get if exposing the lower netdevs?
>>>
>>> Thanks,
>>> -Siwei
>> The only real advantage to a 2 netdev solution is that it looks like
>> the netvsc solution, however it doesn't behave like it since there
are
>> some features like XDP that may not function correctly if they are
>> left enabled in the virtio_net interface.
>>
>> As far as functionality the advantage of not hiding the lower devices
>> is that they are free to be managed. The problem with pushing all of
>> the configuration into the upper device is that you are limited to the
>> intersection of the features of the lower devices. This can be
>> limiting for some setups as some VFs support things like more queues,
>> or better interrupt moderation options than others so trying to make
>> everything work with one config would be ugly.
> It depends on how you build it and the way you expect it to work. IMHO
> the lower devices don't need to be directly managed at all, otherwise
> it ends up with loss of configuration across migration, and it really
> does not bring much value than having a general team or bond device.
> Users still have to reconfigure those queue settings and interrupt
> moderation options after all. The new upper device could take the
> assumption that the VF/PT lower device always has superior feature set
> than virtio-net in order to apply advanced configuration. The upper
> device should remember all configurations previously done and apply
> supporting ones to active device automatically when switching the
> datapath.
>
It should be possible to extend this patchset to support migration of 
additional
settings? by enabling additional ndo_ops and ethtool_ops on the upper dev
and propagating them down the lower devices and replaying the settings after
the VF is replugged after migration.

Thanks
Sridhar

On 2/21/2018 6:35 PM, Samudrala, Sridhar wrote:
> On 2/21/2018 5:59 PM, Siwei Liu wrote:
>> On Wed, Feb 21, 2018 at 4:17 PM, Alexander Duyck
>> <alexander.duyck at gmail.com> wrote:
>>> On Wed, Feb 21, 2018 at 3:50 PM, Siwei Liu <loseweigh at
gmail.com> wrote:
>>>> I haven't checked emails for days and did not realize the
new revision
>>>> had already came out. And thank you for the effort, this
revision
>>>> really looks to be a step forward towards our use case and is
close to
>>>> what we wanted to do. A few questions in line.
>>>>
>>>> On Sat, Feb 17, 2018 at 9:12 AM, Alexander Duyck
>>>> <alexander.duyck at gmail.com> wrote:
>>>>> On Fri, Feb 16, 2018 at 6:38 PM, Jakub Kicinski
<kubakici at wp.pl>
>>>>> wrote:
>>>>>> On Fri, 16 Feb 2018 10:11:19 -0800, Sridhar Samudrala
wrote:
>>>>>>> Ppatch 2 is in response to the community request
for a 3 netdev
>>>>>>> solution.? However, it creates some issues
we'll get into in a
>>>>>>> moment.
>>>>>>> It extends virtio_net to use alternate datapath
when available and
>>>>>>> registered. When BACKUP feature is enabled,
virtio_net driver
>>>>>>> creates
>>>>>>> an additional 'bypass' netdev that acts as
a master device and
>>>>>>> controls
>>>>>>> 2 slave devices.? The original virtio_net netdev is
registered as
>>>>>>> 'backup' netdev and a passthru/vf device
with the same MAC gets
>>>>>>> registered as 'active' netdev. Both
'bypass' and 'backup'
>>>>>>> netdevs are
>>>>>>> associated with the same 'pci' device.? The
user accesses the
>>>>>>> network
>>>>>>> interface via 'bypass' netdev. The
'bypass' netdev chooses
>>>>>>> 'active' netdev
>>>>>>> as default for transmits when it is available with
link up and
>>>>>>> running.
>>>>>> Thank you do doing this.
>>>>>>
>>>>>>> We noticed a couple of issues with this approach
during testing.
>>>>>>> - As both 'bypass' and 'backup'
netdevs are associated with the
>>>>>>> same
>>>>>>> ?? virtio pci device, udev tries to rename both of
them with the
>>>>>>> same name
>>>>>>> ?? and the 2nd rename will fail. This would be OK
as long as the
>>>>>>> first netdev
>>>>>>> ?? to be renamed is the 'bypass' netdev,
but the order in which
>>>>>>> udev gets
>>>>>>> ?? to rename the 2 netdevs is not reliable.
>>>>>> Out of curiosity - why do you link the master netdev to
the virtio
>>>>>> struct device?
>>>>> The basic idea of all this is that we wanted this to work
with an
>>>>> existing VM image that was using virtio. As such we were
trying to
>>>>> make it so that the bypass interface takes the place of the
original
>>>>> virtio and get udev to rename the bypass to what the
original
>>>>> virtio_net was.
>>>> Could it made it also possible to take over the config from VF
instead
>>>> of virtio on an existing VM image? And get udev rename the
bypass
>>>> netdev to what the original VF was. I don't say tightly
binding the
>>>> bypass master to only virtio or VF, but I think we should
provide both
>>>> options to support different upgrade paths. Possibly we could
tweak
>>>> the device tree layout to reuse the same PCI slot for the
master
>>>> bypass netdev, such that udev would not get confused when
renaming the
>>>> device. The VF needs to use a different function slot
afterwards.
>>>> Perhaps we might need to a special multiseat like QEMU device
for that
>>>> purpose?
>>>>
>>>> Our case we'll upgrade the config from VF to virtio-bypass
directly.
>>> So if I am understanding what you are saying you are wanting to
flip
>>> the backup interface from the virtio to a VF. The problem is that
>>> becomes a bit of a vendor lock-in solution since it would rely on a
>>> specific VF driver. I would agree with Jiri that we don't want
to go
>>> down that path. We don't want every VF out there firing up its
own
>>> separate bond. Ideally you want the hypervisor to be able to manage
>>> all of this which is why it makes sense to have virtio manage this
and
>>> why this is associated with the virtio_net interface.
>> No, that's not what I was talking about of course. I thought you
>> mentioned the upgrade scenario this patch would like to address is to
>> use the bypass interface "to take the place of the original
virtio,
>> and get udev to rename the bypass to what the original virtio_net
>> was". That is one of the possible upgrade paths for sure. However
the
>> upgrade path I was seeking is to use the bypass interface to take the
>> place of original VF interface while retaining the name and network
>> configs, which generally can be done simply with kernel upgrade. It
>> would become limiting as this patch makes the bypass interface share
>> the same virtio pci device with virito backup. Can this bypass
>> interface be made general to take place of any pci device other than
>> virtio-net? This will be more helpful as the cloud users who has
>> existing setup on VF interface don't have to recreate it on
virtio-net
>> and VF separately again.
>
> Yes. This sounds interesting. Looks like you want an existing VM image 
> with
> VF only configuration to get transparent live migration support by adding
> virtio_net with BACKUP feature.? We may need another feature bit to 
> switch
> between these 2 options.
After thinking some more, this may be more involved than adding a new
feature bit.? This requires a netdev created by virtio to take over the 
name of
a VF netdev associated with a PCI device that may not be plugged in when
the virtio driver is coming up. This definitely requires some new messages
exchanged across the virtio control queue to pass the PCI device info of the
VF netdev.
>
>
>>
>>> The other bits get into more complexity then we are ready to handle
>>> for now. I think I might have talked about something similar that I
>>> was referring to as a "virtio-bond" where you would have
a PCI/PCIe
>>> tree topology that makes this easier to sort out, and the
"virtio-bond
>>> would be used to handle coordination/configuration of a much more
>>> complex interface.
>> That was one way to solve this problem but I'd like to see simple
ways
>> to sort it out.
>>
>>>>>> FWIW two solutions that immediately come to mind is to
export
>>>>>> "backup"
>>>>>> as phys_port_name of the backup virtio link and/or
assign a name
>>>>>> to the
>>>>>> master like you are doing already.? I think team uses
team%d and
>>>>>> bond
>>>>>> uses bond%d, soft naming of master devices seems quite
natural in
>>>>>> this
>>>>>> case.
>>>>> I figured I had overlooked something like that.. Thanks for
pointing
>>>>> this out. Okay so I think the phys_port_name approach might
resolve
>>>>> the original issue. If I am reading things correctly what
we end up
>>>>> with is the master showing up as "ens1" for
example and the backup
>>>>> showing up as "ens1nbackup". Am I understanding
that right?
>>>>>
>>>>> The problem with the team/bond%d approach is that it
creates a new
>>>>> netdevice and so it would require guest configuration
changes.
>>>>>
>>>>>> IMHO phys_port_name == "backup" if BACKUP bit
is set on slave virtio
>>>>>> link is quite neat.
>>>>> I agree. For non-"backup" virio_net devices would
it be okay for
>>>>> us to
>>>>> just return -EOPNOTSUPP? I assume it would be and that way
the legacy
>>>>> behavior could be maintained although the function still
exists.
>>>>>
>>>>>>> - When the 'active' netdev is unplugged OR
not present on a
>>>>>>> destination
>>>>>>> ?? system after live migration, the user will see 2
virtio_net
>>>>>>> netdevs.
>>>>>> That's necessary and expected, all configuration
applies to the
>>>>>> master
>>>>>> so master must exist.
>>>>> With the naming issue resolved this is the only item left 
>>>>> outstanding.
>>>>> This becomes a matter of form vs function.
>>>>>
>>>>> The main complaint about the "3 netdev" solution
is a bit
>>>>> confusing to
>>>>> have the 2 netdevs present if the VF isn't there. The
idea is that
>>>>> having the extra "master" netdev there if there
isn't really a
>>>>> bond is
>>>>> a bit ugly.
>>>> Is it this uglier in terms of user experience rather than
>>>> functionality? I don't want it dynamically changed between
2-netdev
>>>> and 3-netdev depending on the presence of VF. That gets back to
my
>>>> original question and suggestion earlier: why not just hide the
lower
>>>> netdevs from udev renaming and such? Which important
observability
>>>> benefits users may get if exposing the lower netdevs?
>>>>
>>>> Thanks,
>>>> -Siwei
>>> The only real advantage to a 2 netdev solution is that it looks
like
>>> the netvsc solution, however it doesn't behave like it since
there are
>>> some features like XDP that may not function correctly if they are
>>> left enabled in the virtio_net interface.
>>>
>>> As far as functionality the advantage of not hiding the lower
devices
>>> is that they are free to be managed. The problem with pushing all
of
>>> the configuration into the upper device is that you are limited to
the
>>> intersection of the features of the lower devices. This can be
>>> limiting for some setups as some VFs support things like more
queues,
>>> or better interrupt moderation options than others so trying to
make
>>> everything work with one config would be ugly.
>> It depends on how you build it and the way you expect it to work. IMHO
>> the lower devices don't need to be directly managed at all,
otherwise
>> it ends up with loss of configuration across migration, and it really
>> does not bring much value than having a general team or bond device.
>> Users still have to reconfigure those queue settings and interrupt
>> moderation options after all. The new upper device could take the
>> assumption that the VF/PT lower device always has superior feature set
>> than virtio-net in order to apply advanced configuration. The upper
>> device should remember all configurations previously done and apply
>> supporting ones to active device automatically when switching the
>> datapath.
>>
> It should be possible to extend this patchset to support migration of 
> additional
> settings? by enabling additional ndo_ops and ethtool_ops on the upper dev
> and propagating them down the lower devices and replaying the settings 
> after
> the VF is replugged after migration.
>
> Thanks
> Sridhar

On Wed, Feb 21, 2018 at 6:35 PM, Samudrala, Sridhar
<sridhar.samudrala at intel.com> wrote:
> On 2/21/2018 5:59 PM, Siwei Liu wrote:
>>
>> On Wed, Feb 21, 2018 at 4:17 PM, Alexander Duyck
>> <alexander.duyck at gmail.com> wrote:
>>>
>>> On Wed, Feb 21, 2018 at 3:50 PM, Siwei Liu <loseweigh at
gmail.com> wrote:
>>>>
>>>> I haven't checked emails for days and did not realize the
new revision
>>>> had already came out. And thank you for the effort, this
revision
>>>> really looks to be a step forward towards our use case and is
close to
>>>> what we wanted to do. A few questions in line.
>>>>
>>>> On Sat, Feb 17, 2018 at 9:12 AM, Alexander Duyck
>>>> <alexander.duyck at gmail.com> wrote:
>>>>>
>>>>> On Fri, Feb 16, 2018 at 6:38 PM, Jakub Kicinski
<kubakici at wp.pl> wrote:
>>>>>>
>>>>>> On Fri, 16 Feb 2018 10:11:19 -0800, Sridhar Samudrala
wrote:
>>>>>>>
>>>>>>> Ppatch 2 is in response to the community request
for a 3 netdev
>>>>>>> solution.  However, it creates some issues
we'll get into in a
>>>>>>> moment.
>>>>>>> It extends virtio_net to use alternate datapath
when available and
>>>>>>> registered. When BACKUP feature is enabled,
virtio_net driver creates
>>>>>>> an additional 'bypass' netdev that acts as
a master device and
>>>>>>> controls
>>>>>>> 2 slave devices.  The original virtio_net netdev is
registered as
>>>>>>> 'backup' netdev and a passthru/vf device
with the same MAC gets
>>>>>>> registered as 'active' netdev. Both
'bypass' and 'backup' netdevs are
>>>>>>> associated with the same 'pci' device.  The
user accesses the network
>>>>>>> interface via 'bypass' netdev. The
'bypass' netdev chooses 'active'
>>>>>>> netdev
>>>>>>> as default for transmits when it is available with
link up and
>>>>>>> running.
>>>>>>
>>>>>> Thank you do doing this.
>>>>>>
>>>>>>> We noticed a couple of issues with this approach
during testing.
>>>>>>> - As both 'bypass' and 'backup'
netdevs are associated with the same
>>>>>>>    virtio pci device, udev tries to rename both of
them with the same
>>>>>>> name
>>>>>>>    and the 2nd rename will fail. This would be OK
as long as the
>>>>>>> first netdev
>>>>>>>    to be renamed is the 'bypass' netdev,
but the order in which udev
>>>>>>> gets
>>>>>>>    to rename the 2 netdevs is not reliable.
>>>>>>
>>>>>> Out of curiosity - why do you link the master netdev to
the virtio
>>>>>> struct device?
>>>>>
>>>>> The basic idea of all this is that we wanted this to work
with an
>>>>> existing VM image that was using virtio. As such we were
trying to
>>>>> make it so that the bypass interface takes the place of the
original
>>>>> virtio and get udev to rename the bypass to what the
original
>>>>> virtio_net was.
>>>>
>>>> Could it made it also possible to take over the config from VF
instead
>>>> of virtio on an existing VM image? And get udev rename the
bypass
>>>> netdev to what the original VF was. I don't say tightly
binding the
>>>> bypass master to only virtio or VF, but I think we should
provide both
>>>> options to support different upgrade paths. Possibly we could
tweak
>>>> the device tree layout to reuse the same PCI slot for the
master
>>>> bypass netdev, such that udev would not get confused when
renaming the
>>>> device. The VF needs to use a different function slot
afterwards.
>>>> Perhaps we might need to a special multiseat like QEMU device
for that
>>>> purpose?
>>>>
>>>> Our case we'll upgrade the config from VF to virtio-bypass
directly.
>>>
>>> So if I am understanding what you are saying you are wanting to
flip
>>> the backup interface from the virtio to a VF. The problem is that
>>> becomes a bit of a vendor lock-in solution since it would rely on a
>>> specific VF driver. I would agree with Jiri that we don't want
to go
>>> down that path. We don't want every VF out there firing up its
own
>>> separate bond. Ideally you want the hypervisor to be able to manage
>>> all of this which is why it makes sense to have virtio manage this
and
>>> why this is associated with the virtio_net interface.
>>
>> No, that's not what I was talking about of course. I thought you
>> mentioned the upgrade scenario this patch would like to address is to
>> use the bypass interface "to take the place of the original
virtio,
>> and get udev to rename the bypass to what the original virtio_net
>> was". That is one of the possible upgrade paths for sure. However
the
>> upgrade path I was seeking is to use the bypass interface to take the
>> place of original VF interface while retaining the name and network
>> configs, which generally can be done simply with kernel upgrade. It
>> would become limiting as this patch makes the bypass interface share
>> the same virtio pci device with virito backup. Can this bypass
>> interface be made general to take place of any pci device other than
>> virtio-net? This will be more helpful as the cloud users who has
>> existing setup on VF interface don't have to recreate it on
virtio-net
>> and VF separately again.
>
>
> Yes. This sounds interesting. Looks like you want an existing VM image with
> VF only configuration to get transparent live migration support by adding
> virtio_net with BACKUP feature.  We may need another feature bit to switch
> between these 2 options.
Yes, that's what I was thinking about. I have been building something
like this before, and would like to get back after merging with your
patch.
>
>
>
>>
>>> The other bits get into more complexity then we are ready to handle
>>> for now. I think I might have talked about something similar that I
>>> was referring to as a "virtio-bond" where you would have
a PCI/PCIe
>>> tree topology that makes this easier to sort out, and the
"virtio-bond
>>> would be used to handle coordination/configuration of a much more
>>> complex interface.
>>
>> That was one way to solve this problem but I'd like to see simple
ways
>> to sort it out.
>>
>>>>>> FWIW two solutions that immediately come to mind is to
export "backup"
>>>>>> as phys_port_name of the backup virtio link and/or
assign a name to
>>>>>> the
>>>>>> master like you are doing already.  I think team uses
team%d and bond
>>>>>> uses bond%d, soft naming of master devices seems quite
natural in this
>>>>>> case.
>>>>>
>>>>> I figured I had overlooked something like that.. Thanks for
pointing
>>>>> this out. Okay so I think the phys_port_name approach might
resolve
>>>>> the original issue. If I am reading things correctly what
we end up
>>>>> with is the master showing up as "ens1" for
example and the backup
>>>>> showing up as "ens1nbackup". Am I understanding
that right?
>>>>>
>>>>> The problem with the team/bond%d approach is that it
creates a new
>>>>> netdevice and so it would require guest configuration
changes.
>>>>>
>>>>>> IMHO phys_port_name == "backup" if BACKUP bit
is set on slave virtio
>>>>>> link is quite neat.
>>>>>
>>>>> I agree. For non-"backup" virio_net devices would
it be okay for us to
>>>>> just return -EOPNOTSUPP? I assume it would be and that way
the legacy
>>>>> behavior could be maintained although the function still
exists.
>>>>>
>>>>>>> - When the 'active' netdev is unplugged OR
not present on a
>>>>>>> destination
>>>>>>>    system after live migration, the user will see 2
virtio_net
>>>>>>> netdevs.
>>>>>>
>>>>>> That's necessary and expected, all configuration
applies to the master
>>>>>> so master must exist.
>>>>>
>>>>> With the naming issue resolved this is the only item left
outstanding.
>>>>> This becomes a matter of form vs function.
>>>>>
>>>>> The main complaint about the "3 netdev" solution
is a bit confusing to
>>>>> have the 2 netdevs present if the VF isn't there. The
idea is that
>>>>> having the extra "master" netdev there if there
isn't really a bond is
>>>>> a bit ugly.
>>>>
>>>> Is it this uglier in terms of user experience rather than
>>>> functionality? I don't want it dynamically changed between
2-netdev
>>>> and 3-netdev depending on the presence of VF. That gets back to
my
>>>> original question and suggestion earlier: why not just hide the
lower
>>>> netdevs from udev renaming and such? Which important
observability
>>>> benefits users may get if exposing the lower netdevs?
>>>>
>>>> Thanks,
>>>> -Siwei
>>>
>>> The only real advantage to a 2 netdev solution is that it looks
like
>>> the netvsc solution, however it doesn't behave like it since
there are
>>> some features like XDP that may not function correctly if they are
>>> left enabled in the virtio_net interface.
>>>
>>> As far as functionality the advantage of not hiding the lower
devices
>>> is that they are free to be managed. The problem with pushing all
of
>>> the configuration into the upper device is that you are limited to
the
>>> intersection of the features of the lower devices. This can be
>>> limiting for some setups as some VFs support things like more
queues,
>>> or better interrupt moderation options than others so trying to
make
>>> everything work with one config would be ugly.
>>
>> It depends on how you build it and the way you expect it to work. IMHO
>> the lower devices don't need to be directly managed at all,
otherwise
>> it ends up with loss of configuration across migration, and it really
>> does not bring much value than having a general team or bond device.
>> Users still have to reconfigure those queue settings and interrupt
>> moderation options after all. The new upper device could take the
>> assumption that the VF/PT lower device always has superior feature set
>> than virtio-net in order to apply advanced configuration. The upper
>> device should remember all configurations previously done and apply
>> supporting ones to active device automatically when switching the
>> datapath.
>>
> It should be possible to extend this patchset to support migration of
> additional
> settings  by enabling additional ndo_ops and ethtool_ops on the upper dev
> and propagating them down the lower devices and replaying the settings
after
> the VF is replugged after migration.
Indeed. But your 3rd patch will collapse this merit of the 3-netdev
into the former 2-netdev model - I hope it's just for demostrating the
possibility of dynamically switching ndo_ops and ethtool_ops but not
actually making it complicated in implementing this further.

Thanks,
-Siwei
>
> Thanks
> Sridhar

Fri, Feb 23, 2018 at 11:22:36PM CET, loseweigh at gmail.com wrote:

[...]
>>>
>>> No, that's not what I was talking about of course. I thought
you
>>> mentioned the upgrade scenario this patch would like to address is
to
>>> use the bypass interface "to take the place of the original
virtio,
>>> and get udev to rename the bypass to what the original virtio_net
>>> was". That is one of the possible upgrade paths for sure.
However the
>>> upgrade path I was seeking is to use the bypass interface to take
the
>>> place of original VF interface while retaining the name and network
>>> configs, which generally can be done simply with kernel upgrade. It
>>> would become limiting as this patch makes the bypass interface
share
>>> the same virtio pci device with virito backup. Can this bypass
>>> interface be made general to take place of any pci device other
than
>>> virtio-net? This will be more helpful as the cloud users who has
>>> existing setup on VF interface don't have to recreate it on
virtio-net
>>> and VF separately again.
How that could work? If you have the VF netdev with all configuration
including IPs and routes and whatever - now you want to do migration
so you add virtio_net and do some weird in-driver bonding with it. But
then, VF disappears and the VF netdev with that and also all
configuration it had.
I don't think this scenario is valid.

>>
>>
>> Yes. This sounds interesting. Looks like you want an existing VM image
with
>> VF only configuration to get transparent live migration support by
adding
>> virtio_net with BACKUP feature.  We may need another feature bit to
switch
>> between these 2 options.
>
>Yes, that's what I was thinking about. I have been building something
>like this before, and would like to get back after merging with your
>patch.