Re: [PATCH] vdpa/mlx5: Use random MAC address when no nic vport MAC set

[Cindy Lu]

On Fri, 30 Aug 2024 at 22:46, Dragos Tatulea <dtatulea@xxxxxxxxxx> wrote:
>
> Hi Cindy,
>
> On 30.08.24 15:52, Dragos Tatulea wrote:
> >
> >
> > On 30.08.24 11:12, Cindy Lu wrote:
> >> On Thu, 29 Aug 2024 at 18:00, Dragos Tatulea <dtatulea@xxxxxxxxxx> wrote:
> >>>
> >>>
> >>>
> >>> On 29.08.24 11:05, Cindy Lu wrote:
> >>>> On Wed, 28 Aug 2024 at 17:37, Dragos Tatulea <dtatulea@xxxxxxxxxx> wrote:
> >>>>>
> >>>>>
> >>>>>
> >>>>> On 28.08.24 11:00, Cindy Lu wrote:
> >>>>>> On Wed, 28 Aug 2024 at 09:51, Jason Wang <jasowang@xxxxxxxxxx> wrote:
> >>>>>>>
> >>>>>>> On Wed, Aug 28, 2024 at 12:03 AM Dragos Tatulea <dtatulea@xxxxxxxxxx> wrote:
> >>>>>>>>
> >>>>>>>> When the vdpa device is configured without a specific MAC
> >>>>>>>> address, the vport MAC address is used. However, this
> >>>>>>>> address can be 0 which prevents the driver from properly
> >>>>>>>> configuring the MPFS and breaks steering.
> >>>>>>>>
> >>>>>>>> The solution is to simply generate a random MAC address
> >>>>>>>> when no MAC is set on the nic vport.
> >>>>>>>>
> >>>>>>>> Now it's possible to create a vdpa device without a
> >>>>>>>> MAC address and run qemu with this device without needing
> >>>>>>>> to configure an explicit MAC address.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Dragos Tatulea <dtatulea@xxxxxxxxxx>
> >>>>>>>> Reviewed-by: Jiri Pirko <jiri@xxxxxxxxxx>
> >>>>>>>
> >>>>>>> Acked-by: Jason Wang <jasowang@xxxxxxxxxx>
> >>>>>>>
> >>>>>>> (Adding Cindy for double checking if it has any side effect on Qemu side)
> >>>>>>>
> >>>>>>> Thanks
> >>>>>>>
> >>>>>> But Now there is a bug in QEMU: if the hardware MAC address does not
> >>>>>> match the one in the QEMU command line, it will cause traffic loss.
> >>>>>>
> >>>>> Why is this a new issue in qemu? qemu in it's current state won't work
> >>>>> with a different mac address that the one that is set in HW anyway.
> >>>>>
> >>>> this is not a new bug. We are trying to fix it because it will cause
> >>>> traffic lose without any warning.
> >>>> in my fix , this setting (different mac in device and Qemu) will fail
> >>>> to load the VM.
> >>> Which is a good thing, right? Some feedback to the user that there is
> >>> a misconfig. I got bitten by this so many times... Thank you for adding it.
> >>>
> >>>>
> >>>>>> So, Just an FYI here: if your patch merged, it may cause traffic loss.
> >>>>>> and now I'm working in the fix it in qemu, the link is
> >>>>>> https://patchew.org/QEMU/20240716011349.821777-1-lulu@xxxxxxxxxx/
> >>>>>> The idea of this fix is
> >>>>>> There are will only two acceptable situations for qemu:
> >>>>>> 1. The hardware MAC address is the same as the MAC address specified
> >>>>>> in the QEMU command line, and both MAC addresses are not 0.
> >>>>>> 2. The hardware MAC address is not 0, and the MAC address in the QEMU
> >>>>>> command line is 0. In this situation, the hardware MAC address will
> >>>>>> overwrite the QEMU command line address.
> >>>>>>
> >>>>> Why would this not work with this patch? This patch simply sets a MAC
> >>>>> if the vport doesn't have one set. Which allows for more scenarios to
> >>>>> work.
> >>>>>
> >>>> I do not mean your patch will not work, I just want to make some
> >>>> clarify here.Your patch + my fix may cause the VM to fail to load in
> >>>> some situations, and this is as expected.
> >>>> Your patch is good to merge.
> >>> Ack. Thank you for the clarification.
> >>>
> >>> Thanks,
> >>> Dragos
> >>>
> >> Hi Dragos，
> >>  I think we need to hold this patch. Because it may not be working
> >> with upstream qemu.
> >>
> >> MLX will create a random MAC address for your patch. Additionally, if
> >> there is no specific MAC in the QEMU command line, QEMU will also
> >> generate a random MAC.
> >> these two MAC are not the same. and this will cause traffic loss.
> > Ahaa, it turns out that qemu 8.x and 9.x have different behaviour.
> >
> > Initially I was testing this scenario (vdpa device created with no mac
> > and no mac set in qemu cli) with qemu 8.x. There, qemu was not being
> > able to set the qemu generated random mac addres because .set_config()
> > is a nop in mlx5_vdpa.
> >
> > Then I moved to qemu 9.x and saw that this scenario was working because
> > now the CVQ was used instead to configure the mac on the device.
> >
> > So this patch should definitely not be applied.
> >
> > I was thinking if there are ways to fix this for 8.x. The only feasible
> > way is to implement .set_config() in mlx5_vdpa for the mac
> > configuration. But as you previousy said, this is discouraged.
> >
> I just tested your referenced qemu fix from patchwork and I found that
> for the case when a vdpa device doesn't have a mac address (mac address
> 0 and VIRTIO_NET_F_MAC not set) qemu will return an error. So with this
> fix we'd be back to square one where the user always has to set a mac
> somewhere.
>
> Would it be possible to take this case into consideration with your
> fix?
>
> Thanks,
> Dragos
>
Hi Dragos

Thanks for your test and help, I think I can add a check for
VIRTIO_NET_F_MAC in the qemu code. if the device's Mac is 0 and the
VIRTIO_NET_F_MAC is not set. The guest VM will fail to load. I will
double-check this
Thanks

Cindy