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

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Message-ID: <CACLfguXM97JAdWcYO17+H6pu7MWLu2QqBgu_PypGxU_Ab+OUOQ@mail.gmail.com>
Date: Fri, 30 Aug 2024 17:29:14 +0800
From: Cindy Lu <lulu@...hat.com>
To: Dragos Tatulea <dtatulea@...dia.com>
Cc: Jason Wang <jasowang@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez <eperezma@...hat.com>, 
	si-wei.liu@...cle.com, Jiri Pirko <jiri@...dia.com>, virtualization@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vdpa/mlx5: Use random MAC address when no nic vport MAC set

On Fri, 30 Aug 2024 at 03:03, Dragos Tatulea <dtatulea@...dia.com> wrote:
>
>
>
> On 29.08.24 12:00, Dragos Tatulea wrote:
> >
> >
> > On 29.08.24 11:05, Cindy Lu wrote:
> >> On Wed, 28 Aug 2024 at 17:37, Dragos Tatulea <dtatulea@...dia.com> wrote:
> >>>
> >>>
> >>>
> >>> On 28.08.24 11:00, Cindy Lu wrote:
> >>>> On Wed, 28 Aug 2024 at 09:51, Jason Wang <jasowang@...hat.com> wrote:
> >>>>>
> >>>>> On Wed, Aug 28, 2024 at 12:03 AM Dragos Tatulea <dtatulea@...dia.com> 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@...dia.com>
> >>>>>> Reviewed-by: Jiri Pirko <jiri@...dia.com>
> >>>>>
> >>>>> Acked-by: Jason Wang <jasowang@...hat.com>
> >>>>>
> >>>>> (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@redhat.com/
> >>>> 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.
> (Side note)
> While looking at another issue I discovered that it's possible to
> configure a random MAC on the mlx5_vdpa device at VM boot time if
> device MAC configuration is implemented during during .set_config(). So
> I was able to boot up a VM with a random MAC address coming from qemu
> and the traffic worked with this new MAC.
>
> So now I'm not sure if this is just by luck or if the .set_config()
> op should be implemented for the MAC part in our device.
>
> Thanks,
> Dragos
>
Hi Dragos，
For qemu part, I think this is not set from set_config()?  it should
be from the CVQ?
Usually, we don't recommend using the set_config() function because
the configuration space should be read-only for modern devices.

Now there is a bug in this part of qemu, and we plan to remove the
code to set_config() in virtio_net_device_realize(), here is the patch
https://lore.kernel.org/all/CACGkMEvCSKfahpBQLAMmSzdFN-QPhg5Zx+UQVrFX0HsWybZZNA@mail.gmail.com/T/
and this is still under review

thanks
cindy