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

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Message-ID: <CACLfguWC-gGoY3JmMA-jU1W7OgkAEQ9iJ8TpZbRtE2iJ5++GxQ@mail.gmail.com>
Date: Mon, 2 Sep 2024 17:55:28 +0800
From: Cindy Lu <lulu@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Dragos Tatulea <dtatulea@...dia.com>, Jason Wang <jasowang@...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 Mon, 2 Sept 2024 at 02:49, Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Fri, Aug 30, 2024 at 05:29:14PM +0800, Cindy Lu wrote:
> > 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?
>
>
> You are confusing two things.
> Provisioning is not through CVQ.
>
Thanks, Michael. I'm sorry for the confusion. I just meant that maybe
the MAC address is set through CVQ. I will make it clearer next time.
Thanks
Cindy
>
> > 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
>