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

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Message-ID: <CACLfguUZVDGaY4MD+_tDqM9DQC-C6cuPfCf34X59e2RkMztEkA@mail.gmail.com>
Date: Fri, 30 Aug 2024 17:12:01 +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 Thu, 29 Aug 2024 at 18:00, Dragos Tatulea <dtatulea@...dia.com> 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.
>
> 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.
As I mentioned before, this is a bug in QEMU, and I'm working on fixing it.

Thanks
Cindy