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

On Mon, Jun 25, 2018 at 6:50 PM, Michael S. Tsirkin <mst at redhat.com>
wrote:
> On Mon, Jun 25, 2018 at 10:54:09AM -0700, Samudrala, Sridhar wrote:
>> > > > > Might not neccessarily be something wrong, but
it's very limited to
>> > > > > prohibit the MAC of VF from changing when enslaved
by failover.
>> > > > You mean guest changing MAC? I'm not sure why we
prohibit that.
>> > > I think Sridhar and Jiri might be better person to answer it.
My
>> > > impression was that sync'ing the MAC address change
between all 3
>> > > devices is challenging, as the failover driver uses MAC
address to
>> > > match net_device internally.
>>
>> Yes. The MAC address is assigned by the hypervisor and it needs to
manage the movement
>> of the MAC between the PF and VF.  Allowing the guest to change the MAC
will require
>> synchronization between the hypervisor and the PF/VF drivers. Most of
the VF drivers
>> don't allow changing guest MAC unless it is a trusted VF.
>
> OK but it's a policy thing. Maybe it's a trusted VF. Who knows?
> For example I can see host just
> failing VIRTIO_NET_CTRL_MAC_ADDR_SET if it wants to block it.
> I'm not sure why VIRTIO_NET_F_STANDBY has to block it in the guest.
That's why I think pairing using MAC is fragile IMHO. When VF's MAC
got changed before virtio attempts to match and pair the device, it
ends up with no pairing found out at all. UUID is better.

-Siwei
>
> --
> MST

On Tue, Jun 26, 2018 at 04:38:26PM -0700, Siwei Liu
wrote:
> On Mon, Jun 25, 2018 at 6:50 PM, Michael S. Tsirkin <mst at
redhat.com> wrote:
> > On Mon, Jun 25, 2018 at 10:54:09AM -0700, Samudrala, Sridhar wrote:
> >> > > > > Might not neccessarily be something wrong, but
it's very limited to
> >> > > > > prohibit the MAC of VF from changing when
enslaved by failover.
> >> > > > You mean guest changing MAC? I'm not sure why
we prohibit that.
> >> > > I think Sridhar and Jiri might be better person to
answer it. My
> >> > > impression was that sync'ing the MAC address change
between all 3
> >> > > devices is challenging, as the failover driver uses MAC
address to
> >> > > match net_device internally.
> >>
> >> Yes. The MAC address is assigned by the hypervisor and it needs to
manage the movement
> >> of the MAC between the PF and VF.  Allowing the guest to change
the MAC will require
> >> synchronization between the hypervisor and the PF/VF drivers. Most
of the VF drivers
> >> don't allow changing guest MAC unless it is a trusted VF.
> >
> > OK but it's a policy thing. Maybe it's a trusted VF. Who
knows?
> > For example I can see host just
> > failing VIRTIO_NET_CTRL_MAC_ADDR_SET if it wants to block it.
> > I'm not sure why VIRTIO_NET_F_STANDBY has to block it in the
guest.
> 
> That's why I think pairing using MAC is fragile IMHO. When VF's MAC
> got changed before virtio attempts to match and pair the device, it
> ends up with no pairing found out at all.
Guest seems to match on the hardware mac and ignore whatever
is set by user. Makes sense to me and should not be fragile.

> UUID is better.
> 
> -Siwei
> 
> >
> > --
> > MST

On Tue, Jun 26, 2018 at 5:29 PM, Michael S. Tsirkin <mst at redhat.com>
wrote:
> On Tue, Jun 26, 2018 at 04:38:26PM -0700, Siwei Liu wrote:
>> On Mon, Jun 25, 2018 at 6:50 PM, Michael S. Tsirkin <mst at
redhat.com> wrote:
>> > On Mon, Jun 25, 2018 at 10:54:09AM -0700, Samudrala, Sridhar
wrote:
>> >> > > > > Might not neccessarily be something wrong,
but it's very limited to
>> >> > > > > prohibit the MAC of VF from changing when
enslaved by failover.
>> >> > > > You mean guest changing MAC? I'm not sure
why we prohibit that.
>> >> > > I think Sridhar and Jiri might be better person to
answer it. My
>> >> > > impression was that sync'ing the MAC address
change between all 3
>> >> > > devices is challenging, as the failover driver uses
MAC address to
>> >> > > match net_device internally.
>> >>
>> >> Yes. The MAC address is assigned by the hypervisor and it
needs to manage the movement
>> >> of the MAC between the PF and VF.  Allowing the guest to
change the MAC will require
>> >> synchronization between the hypervisor and the PF/VF drivers.
Most of the VF drivers
>> >> don't allow changing guest MAC unless it is a trusted VF.
>> >
>> > OK but it's a policy thing. Maybe it's a trusted VF. Who
knows?
>> > For example I can see host just
>> > failing VIRTIO_NET_CTRL_MAC_ADDR_SET if it wants to block it.
>> > I'm not sure why VIRTIO_NET_F_STANDBY has to block it in the
guest.
>>
>> That's why I think pairing using MAC is fragile IMHO. When VF's
MAC
>> got changed before virtio attempts to match and pair the device, it
>> ends up with no pairing found out at all.
>
> Guest seems to match on the hardware mac and ignore whatever
> is set by user. Makes sense to me and should not be fragile.
Host can change the hardware mac for VF any time.

-Siwei
>
>
>> UUID is better.
>>
>> -Siwei
>>
>> >
>> > --
>> > MST