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

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Message-ID: <CACLfguVr1bd6=bkGn6hX3W7xBr45qydaCpQ1mNpsATeWFqe2ZA@mail.gmail.com>
Date: Thu, 29 Aug 2024 17:05:38 +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 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.

> > 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.
Thanks
cindy

> Thanks,
> Dragos
>
> > Thanks
> > Cindy
> >
> >
> >>> ---
> >>>  drivers/vdpa/mlx5/net/mlx5_vnet.c | 3 +++
> >>>  1 file changed, 3 insertions(+)
> >>>
> >>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> >>> index fa78e8288ebb..1c26139d02fe 100644
> >>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> >>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> >>> @@ -3824,6 +3824,9 @@ static int mlx5_vdpa_dev_add(struct vdpa_mgmt_dev *v_mdev, const char *name,
> >>>                 err = mlx5_query_nic_vport_mac_address(mdev, 0, 0, config->mac);
> >>>                 if (err)
> >>>                         goto err_alloc;
> >>> +
> >>> +               if (is_zero_ether_addr(config->mac))
> >>> +                       eth_random_addr(config->mac);
> >>>         }
> >>>
> >>>         if (!is_zero_ether_addr(config->mac)) {
> >>> --
> >>> 2.45.1
> >>>
> >>
> >
>