Date:   Wed, 2 Dec 2020 08:04:48 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Cindy Lu <lulu@...hat.com>, Eli Cohen <elic@...dia.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vdpa/mlx5: Use random MAC for the vdpa net instance

On Wed, Dec 02, 2020 at 08:56:37PM +0800, Jason Wang wrote:
> 
> On 2020/12/2 下午5:30, Michael S. Tsirkin wrote:
> > On Wed, Dec 02, 2020 at 12:18:36PM +0800, Jason Wang wrote:
> > > On 2020/12/1 下午5:23, Cindy Lu wrote:
> > > > On Mon, Nov 30, 2020 at 11:33 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > > > On Mon, Nov 30, 2020 at 06:41:45PM +0800, Cindy Lu wrote:
> > > > > > On Mon, Nov 30, 2020 at 5:33 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > > > > > On Mon, Nov 30, 2020 at 11:27:59AM +0200, Eli Cohen wrote:
> > > > > > > > On Mon, Nov 30, 2020 at 04:00:51AM -0500, Michael S. Tsirkin wrote:
> > > > > > > > > On Mon, Nov 30, 2020 at 08:27:46AM +0200, Eli Cohen wrote:
> > > > > > > > > > On Sun, Nov 29, 2020 at 03:08:22PM -0500, Michael S. Tsirkin wrote:
> > > > > > > > > > > On Sun, Nov 29, 2020 at 08:43:51AM +0200, Eli Cohen wrote:
> > > > > > > > > > > > We should not try to use the VF MAC address as that is used by the
> > > > > > > > > > > > regular (e.g. mlx5_core) NIC implementation. Instead, use a random
> > > > > > > > > > > > generated MAC address.
> > > > > > > > > > > > 
> > > > > > > > > > > > Suggested by: Cindy Lu <lulu@...hat.com>
> > > > > > > > > > > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
> > > > > > > > > > > > Signed-off-by: Eli Cohen <elic@...dia.com>
> > > > > > > > > > > I didn't realise it's possible to use VF in two ways
> > > > > > > > > > > with and without vdpa.
> > > > > > > > > > Using a VF you can create quite a few resources, e.g. send queues
> > > > > > > > > > recieve queues, virtio_net queues etc. So you can possibly create
> > > > > > > > > > several instances of vdpa net devices and nic net devices.
> > > > > > > > > > 
> > > > > > > > > > > Could you include a bit more description on the failure
> > > > > > > > > > > mode?
> > > > > > > > > > Well, using the MAC address of the nic vport is wrong since that is the
> > > > > > > > > > MAC of the regular NIC implementation of mlx5_core.
> > > > > > > > > Right but ATM it doesn't coexist with vdpa so what's the problem?
> > > > > > > > > 
> > > > > > > > This call is wrong:  mlx5_query_nic_vport_mac_address()
> > > > > > > > 
> > > > > > > > > > > Is switching to a random mac for such an unusual
> > > > > > > > > > > configuration really justified?
> > > > > > > > > > Since I can't use the NIC's MAC address, I have two options:
> > > > > > > > > > 1. To get the MAC address as was chosen by the user administering the
> > > > > > > > > >      NIC. This should invoke the set_config callback. Unfortunately this
> > > > > > > > > >      is not implemented yet.
> > > > > > > > > > 
> > > > > > > > > > 2. Use a random MAC address. This is OK since if (1) is implemented it
> > > > > > > > > >      can always override this random configuration.
> > > > > > > > > > 
> > > > > > > > > > > It looks like changing a MAC could break some guests,
> > > > > > > > > > > can it not?
> > > > > > > > > > > 
> > > > > > > > > > No, it will not. The current version of mlx5 VDPA does not allow regular
> > > > > > > > > > NIC driver and VDPA to co-exist. I have patches ready that enable that
> > > > > > > > > > from steering point of view. I will post them here once other patches on
> > > > > > > > > > which they depend will be merged.
> > > > > > > > > > 
> > > > > > > > > > https://patchwork.ozlabs.org/project/netdev/patch/20201120230339.651609-12-saeedm@nvidia.com/
> > > > > > > > > Could you be more explicit on the following points:
> > > > > > > > > - which configuration is broken ATM (as in, two device have identical
> > > > > > > > >     macs? any other issues)?
> > > > > > > > The only wrong thing is the call to  mlx5_query_nic_vport_mac_address().
> > > > > > > > It's not breaking anything yet is wrong. The random MAC address setting
> > > > > > > > is required for the steering patches.
> > > > > > > Okay so I'm not sure the Fixes tag at least is appropriate if it's a
> > > > > > > dependency of a new feature.
> > > > > > > 
> > > > > > > > > - why won't device MAC change from guest point of view?
> > > > > > > > > 
> > > > > > > > It's lack of implementation in qemu as far as I know.
> > > > > > > Sorry not sure I understand. What's not implemented in QEMU?
> > > > > > > 
> > > > > > HI Michael, there are some bug in qemu to set_config, this will fix in future,
> > > > > > But this patch is still needed, because without this patch the mlx
> > > > > > driver will give an 0 mac address to qemu
> > > > > > and qemu will overwrite the default mac address.  This will cause traffic down.
> > > > > Hmm the patch description says VF mac address, not 0 address. Confused.
> > > > > If there's no mac we can clear VIRTIO_NET_F_MAC and have guest
> > > > > use a random value ...
> > > 
> > > I'm not sure this can work for all types of vDPA (e.g it could not be a
> > > learning bridge in the swtich).
> > > 
> > > 
> > > > hi Michael，
> > > > I have tried as your suggestion, seems even remove the
> > > > VIRTIO_NET_F_MAC the qemu will still call get_cinfig and overwrite the
> > > > default address in  VM,
> > > 
> > > This looks a bug in qemu, in guest driver we had:
> > > 
> > >      /* Configuration may specify what MAC to use.  Otherwise random. */
> > >      if (virtio_has_feature(vdev, VIRTIO_NET_F_MAC))
> > >          virtio_cread_bytes(vdev,
> > >                     offsetof(struct virtio_net_config, mac),
> > >                     dev->dev_addr, dev->addr_len);
> > >      else
> > >          eth_hw_addr_random(dev);
> > > 
> > > 
> > > > this process is like
> > > > vdpa _init -->qemu call get_config ->mlx driver will give  an mac
> > > > address with all 0-->
> > > > qemu will not check this mac address and use it --> overwrite the mac
> > > > address in qemu
> > > > 
> > > > So for my understanding there are several method to fix this problem
> > > > 
> > > > 1, qemu check the mac address, if the mac address is all 0, qemu will
> > > > ignore it and set the random mac address to mlx driver.
> > > 
> > > So my understanding is that, if mac address is all 0, vDPA parent should not
> > > advertise VIRTIO_NET_F_MAC. And qemu should emulate this feature as you did:
> > > 
> > > 1) get a random mac
> > To me this looks like a spec violation.
> > 
> > If the driver negotiates the VIRTIO_NET_F_MAC feature, the driver MUST set
> > the physical address of the NIC to \field{mac}.  Otherwise, it SHOULD
> > use a locally-administered MAC address (see \hyperref[intro:IEEE 802]{IEEE 802},
> > ``9.2 48-bit universal LAN MAC addresses'').
> 
> 
> One question here, what did "set" mean here consider the mac is given by the
> device itself?
> 


That is my understanding, and this seems to be what linux guests do.

> > 
> > While not said explicitly, the assumption I think is that the local
> > MAC is not a local one.
> > 
> > 
> > > 2) advertise VIRTIO_NET_F_MAC
> > > 3) set the random mac to vDPA through set_config
> > that part looks wrong to me. Setting mac through set_config was
> > a pre-virtio-1.0 way to send mac to device. In 1.0 we have
> > VIRTIO_NET_CTRL_MAC_ADDR_SET for that:
> > 
> > 
> > 	When using the legacy interface, \field{mac} is driver-writable
> > 	which provided a way for drivers to update the MAC without
> > 	negotiating VIRTIO_NET_F_CTRL_MAC_ADDR.
> 
> 
> Looks like it doesn't prevent us from doing so.

>From writing into mac?
Yes it does:
	Device configuration fields are listed below, they are read-only for a driver. The \field{mac} address field
	always exists (though is only valid if VIRTIO_NET_F_MAC is set), and
	\field{status} only exists if VIRTIO_NET_F_STATUS is set.


> Otherwise this brings an
> implicit dependency for control virtqueue if we want to support 1.0?
> 
> Thanks

With 1.0 you either need VIRTIO_NET_F_CTRL_MAC_ADDR or VIRTIO_NET_F_MAC.


> 
> > 
> > 
> > 
> > > 4) advertise the random mac to emulated config to guest
> > > 
> > > 
> > > > 2. mlx driver checks the mac address and if this mac is 0, return fail
> > > > to qemu, but this need to change the UAPI.
> > > 
> > > uAPI is probably fine since ioctl can fail.  We can change the to allow the
> > > set_config to fail but virito spec doesn't have a way to advertise the error
> > > in this case. Anyway, the driver only risk itself for setting a wrong value,
> > > so we're probably fine.
> > > 
> > > Thanks
> > > 
> > > 
> > > > 3. mlx driver it shelf should get an correct mac address while it init.
> > > > 4. add check in qemu get_config function  , if there is not F_MAC Then
> > > > ignore the mac address from mlx driver
> > > > 
> > > > not sure which method is more suitable ?
> > > > 
> > > > Thanks
> > > > Cindy
> > > > 
> > > > 
> > > > 
> > > > > > > > > > > > ---
> > > > > > > > > > > >    drivers/vdpa/mlx5/net/mlx5_vnet.c | 5 +----
> > > > > > > > > > > >    1 file changed, 1 insertion(+), 4 deletions(-)
> > > > > > > > > > > > 
> > > > > > > > > > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > index 1fa6fcac8299..80d06d958b8b 100644
> > > > > > > > > > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > > > > > > @@ -1955,10 +1955,7 @@ void *mlx5_vdpa_add_dev(struct mlx5_core_dev *mdev)
> > > > > > > > > > > >         if (err)
> > > > > > > > > > > >                 goto err_mtu;
> > > > > > > > > > > > 
> > > > > > > > > > > > -     err = mlx5_query_nic_vport_mac_address(mdev, 0, 0, config->mac);
> > > > > > > > > > > > -     if (err)
> > > > > > > > > > > > -             goto err_mtu;
> > > > > > > > > > > > -
> > > > > > > > > > > > +     eth_random_addr(config->mac);
> > > > > > > > > > > >         mvdev->vdev.dma_dev = mdev->device;
> > > > > > > > > > > >         err = mlx5_vdpa_alloc_resources(&ndev->mvdev);
> > > > > > > > > > > >         if (err)
> > > > > > > > > > > > --
> > > > > > > > > > > > 2.26.2

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