Re: VIRTIO_NET_F_MTU not negotiated

Re: VIRTIO_NET_F_MTU not negotiated

[Michael S. Tsirkin]

On Tue, Jul 19, 2022 at 01:25:42PM +0000, Eli Cohen wrote:
> Hi,
> 
>  
> 
> mlx5_vdpa is offering VIRTIO_NET_F_MTU. However the driver (is it qemu
> responsibility?) does not accept and it ends up not negotiated.

qemu is responsible for passing features to driver.

>  
> 
> I don't see how can the driver refuse to negotiate this. What if the hardware
> has a limitation with respect to mtu?

Then it can fail FEATURES_OK

>  
> 
> I noticed this when I created the device with mtu of 1000. I expected the
> netdev at the vm to have mtu of 1000 and any attempt to go beyond should fail
> but that's not the case.
> 
>  
> 
> Comments?


Any chance mtu is too small?

        if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
                int mtu = virtio_cread16(vdev,
                                         offsetof(struct virtio_net_config,
                                                  mtu));
                if (mtu < MIN_MTU)
                        __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
        }

any chance it's on power or another BE system?

-- 
MST

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Re: VIRTIO_NET_F_MTU not negotiated

[Michael S. Tsirkin]

On Wed, Jul 20, 2022 at 08:17:29AM +0000, Eli Cohen wrote:
> > From: Eugenio Perez Martin <eperezma@redhat.com>
> > Sent: Wednesday, July 20, 2022 10:47 AM
> > To: Eli Cohen <elic@nvidia.com>
> > Cc: Michael S. Tsirkin <mst@redhat.com>; Jason Wang <jasowang@redhat.com>; virtualization@lists.linux-foundation.org
> > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > 
> > On Wed, Jul 20, 2022 at 8:30 AM Eli Cohen <elic@nvidia.com> wrote:
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: Eugenio Perez Martin <eperezma@redhat.com>
> > > > Sent: Tuesday, July 19, 2022 5:51 PM
> > > > To: Eli Cohen <elic@nvidia.com>
> > > > Cc: Michael S. Tsirkin <mst@redhat.com>; Jason Wang <jasowang@redhat.com>; virtualization@lists.linux-foundation.org
> > > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > > >
> > > > On Tue, Jul 19, 2022 at 4:02 PM Eli Cohen <elic@nvidia.com> wrote:
> > > > >
> > > > > > From: Eli Cohen
> > > > > > Sent: Tuesday, July 19, 2022 4:53 PM
> > > > > > To: Michael S. Tsirkin <mst@redhat.com>
> > > > > > Cc: Jason Wang <jasowang@redhat.com>; Eugenio Perez Martin <eperezma@redhat.com>; virtualization@lists.linux-
> > > > foundation.org
> > > > > > Subject: RE: VIRTIO_NET_F_MTU not negotiated
> > > > > >
> > > > > > >
> > > > > > > On Tue, Jul 19, 2022 at 01:25:42PM +0000, Eli Cohen wrote:
> > > > > > > > Hi,
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > > mlx5_vdpa is offering VIRTIO_NET_F_MTU. However the driver (is it qemu
> > > > > > > > responsibility?) does not accept and it ends up not negotiated.
> > > > > > >
> > > > > > > qemu is responsible for passing features to driver.
> > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > > I don't see how can the driver refuse to negotiate this. What if the hardware
> > > > > > > > has a limitation with respect to mtu?
> > > > > > >
> > > > > > > Then it can fail FEATURES_OK
> > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > > I noticed this when I created the device with mtu of 1000. I expected the
> > > > > > > > netdev at the vm to have mtu of 1000 and any attempt to go beyond should fail
> > > > > > > > but that's not the case.
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > > Comments?
> > > > > > >
> > > > > > >
> > > > > > > Any chance mtu is too small?
> > > > > > >
> > > > > >
> > > > > > MIN_MTU is 68 bytes and I was trying 1000. I tried also 1300 but same thing.
> > > > > >
> > > > > > >         if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
> > > > > > >                 int mtu = virtio_cread16(vdev,
> > > > > > >                                          offsetof(struct virtio_net_config,
> > > > > > >                                                   mtu));
> > > > > > >                 if (mtu < MIN_MTU)
> > > > > > >                         __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
> > > > > > >         }
> > > > > > >
> > > > > > > any chance it's on power or another BE system?
> > > > > > >
> > > > > >
> > > > > > No, it's x86_64.
> > > > > >
> > > > > > I will test also vdpa device running on the host.
> > > > >
> > > > > vdpa running on the host (using virtio_vdpa) behaves as expected.
> > > > > Is there a quick way to check if qemu fails to pass the information to the driver on the guest?
> > > > >
> > > >
> > > > Can you trace with "-trace 'vhost_vdpa_set_features' -trace
> > > > 'vhost_vdpa_set_features'"? They're parameters of qemu.
> > >
> > > I get this:
> > > vhost_vdpa_set_features dev: 0x5595ae9751a0 features: 0x3008b1803
> > >
> > > VIRTIO_NET_F_MTU is bit 3 and it seems to be off.
> > >
> > 
> > Sorry, I put trace on vhost_vdpa *_set_* features twice in my message.
> > 
> > Could you try tracing vhost_vdpa_get_features too? That way we know if
> > qemu offers it to the guest.
> > 
> 
> Sure.
> I get these two successive prints right as the VM starts booting:
> vhost_vdpa_get_features dev: 0x55c87e4651e0 features: 0x300cb182b
> vhost_vdpa_get_features dev: 0x55c87e4627a0 features: 0x300cb182b
> 
> Later on I get this:
> vhost_vdpa_set_features dev: 0x55c87e4651e0 features: 0x3008b1803
> 
> # qemu-system-x86_64 --version
> QEMU emulator version 7.0.0 (v7.0.0)
> Copyright (c) 2003-2022 Fabrice Bellard and the QEMU Project developers
> 
> 
> 
> > Thanks!
> 

so it's there but driver does not ack it.

-- 
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RE: VIRTIO_NET_F_MTU not negotiated

[Eli Cohen]

I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device (e.g. through libvirt <mtu size="9000"/>).
Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do it using its copy of virtio_net_config.

The code in qemu that is responsible to allow to consider the device MTU restriction is here:

static void virtio_net_device_realize(DeviceState *dev, Error **errp)
{
    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
    VirtIONet *n = VIRTIO_NET(dev);
    NetClientState *nc;
    int i;

    if (n->net_conf.mtu) {
        n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
    }

The above code can be interpreted as follows:
if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the device (that actual value is ignored).

I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that we should read the actual limitation for the device.

If this makes sense I can send a patch to fix this.

Re: VIRTIO_NET_F_MTU not negotiated

[Michael S. Tsirkin]

On Wed, Jul 27, 2022 at 06:51:56AM +0000, Eli Cohen wrote:
> I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device (e.g. through libvirt <mtu size="9000"/>).
> Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do it using its copy of virtio_net_config.
> 
> The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> 
> static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> {
>     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>     VirtIONet *n = VIRTIO_NET(dev);
>     NetClientState *nc;
>     int i;
> 
>     if (n->net_conf.mtu) {
>         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
>     }
> 
> The above code can be interpreted as follows:
> if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the device (that actual value is ignored).
> 
> I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that we should read the actual limitation for the device.
> 
> If this makes sense I can send a patch to fix this.

Well it will then either have to be for vdpa only, or have
compat machinery to avoid breaking migration.

-- 
MST

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Virtualization mailing list
Virtualization@lists.linux-foundation.org
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Re: VIRTIO_NET_F_MTU not negotiated

[Michael S. Tsirkin]

On Wed, Jul 27, 2022 at 06:51:56AM +0000, Eli Cohen wrote:
> I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device (e.g. through libvirt <mtu size="9000"/>).
> Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do it using its copy of virtio_net_config.
> 
> The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> 
> static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> {
>     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>     VirtIONet *n = VIRTIO_NET(dev);
>     NetClientState *nc;
>     int i;
> 
>     if (n->net_conf.mtu) {
>         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
>     }
> 
> The above code can be interpreted as follows:
> if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the device (that actual value is ignored).
> 
> I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that we should read the actual limitation for the device.
> 
> If this makes sense I can send a patch to fix this.

Well it will then either have to be for vdpa only, or have
compat machinery to avoid breaking migration.

-- 
MST

RE: VIRTIO_NET_F_MTU not negotiated

[Eli Cohen]

> -----Original Message-----
> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Wednesday, July 27, 2022 10:25 AM
> To: Eli Cohen <elic@nvidia.com>
> Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> virtualization@lists.linux-foundation.org
> Subject: Re: VIRTIO_NET_F_MTU not negotiated
> 
> On Wed, Jul 27, 2022 at 06:51:56AM +0000, Eli Cohen wrote:
> > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device
> (e.g. through libvirt <mtu size="9000"/>).
> > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do
> it using its copy of virtio_net_config.
> >
> > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> >
> > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > {
> >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> >     VirtIONet *n = VIRTIO_NET(dev);
> >     NetClientState *nc;
> >     int i;
> >
> >     if (n->net_conf.mtu) {
> >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> >     }
> >
> > The above code can be interpreted as follows:
> > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> device (that actual value is ignored).
> >
> > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that
> we should read the actual limitation for the device.
> >
> > If this makes sense I can send a patch to fix this.
> 
> Well it will then either have to be for vdpa only, or have
> compat machinery to avoid breaking migration.
> 

How about this one:

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 1067e72b3975..e464e4645c79 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -3188,6 +3188,7 @@ static void virtio_net_guest_notifier_mask(VirtIODevice *vdev, int idx,
 static void virtio_net_set_config_size(VirtIONet *n, uint64_t host_features)
 {
     virtio_add_feature(&host_features, VIRTIO_NET_F_MAC);
+    virtio_add_feature(&host_features, VIRTIO_NET_F_MTU);

     n->config_size = virtio_feature_get_config_size(feature_sizes,
                                                     host_features);
@@ -3512,6 +3513,7 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)

    if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
         struct virtio_net_config netcfg = {};
+        n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
         memcpy(&netcfg.mac, &n->nic_conf.macaddr, ETH_ALEN);
         vhost_net_set_config(get_vhost_net(nc->peer),
             (uint8_t *)&netcfg, 0, ETH_ALEN, VHOST_SET_CONFIG_TYPE_MASTER);

Re: VIRTIO_NET_F_MTU not negotiated

[Michael S. Tsirkin]

On Wed, Jul 27, 2022 at 09:04:47AM +0000, Eli Cohen wrote:
> > -----Original Message-----
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Wednesday, July 27, 2022 10:25 AM
> > To: Eli Cohen <elic@nvidia.com>
> > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > virtualization@lists.linux-foundation.org
> > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > 
> > On Wed, Jul 27, 2022 at 06:51:56AM +0000, Eli Cohen wrote:
> > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device
> > (e.g. through libvirt <mtu size="9000"/>).
> > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do
> > it using its copy of virtio_net_config.
> > >
> > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > >
> > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > {
> > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > >     VirtIONet *n = VIRTIO_NET(dev);
> > >     NetClientState *nc;
> > >     int i;
> > >
> > >     if (n->net_conf.mtu) {
> > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > >     }
> > >
> > > The above code can be interpreted as follows:
> > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> > device (that actual value is ignored).
> > >
> > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that
> > we should read the actual limitation for the device.
> > >
> > > If this makes sense I can send a patch to fix this.
> > 
> > Well it will then either have to be for vdpa only, or have
> > compat machinery to avoid breaking migration.
> > 
> 
> How about this one:
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 1067e72b3975..e464e4645c79 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -3188,6 +3188,7 @@ static void virtio_net_guest_notifier_mask(VirtIODevice *vdev, int idx,
>  static void virtio_net_set_config_size(VirtIONet *n, uint64_t host_features)
>  {
>      virtio_add_feature(&host_features, VIRTIO_NET_F_MAC);
> +    virtio_add_feature(&host_features, VIRTIO_NET_F_MTU);
> 
>      n->config_size = virtio_feature_get_config_size(feature_sizes,
>                                                      host_features);

Seems to increase config size unconditionally?

> @@ -3512,6 +3513,7 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> 
>     if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
>          struct virtio_net_config netcfg = {};
> +        n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
>          memcpy(&netcfg.mac, &n->nic_conf.macaddr, ETH_ALEN);
>          vhost_net_set_config(get_vhost_net(nc->peer),
>              (uint8_t *)&netcfg, 0, ETH_ALEN, VHOST_SET_CONFIG_TYPE_MASTER);

And the point is vdpa does not support migration anyway ATM, right?

-- 
MST

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Re: VIRTIO_NET_F_MTU not negotiated

[Michael S. Tsirkin]

On Wed, Jul 27, 2022 at 09:04:47AM +0000, Eli Cohen wrote:
> > -----Original Message-----
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Wednesday, July 27, 2022 10:25 AM
> > To: Eli Cohen <elic@nvidia.com>
> > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > virtualization@lists.linux-foundation.org
> > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > 
> > On Wed, Jul 27, 2022 at 06:51:56AM +0000, Eli Cohen wrote:
> > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device
> > (e.g. through libvirt <mtu size="9000"/>).
> > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do
> > it using its copy of virtio_net_config.
> > >
> > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > >
> > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > {
> > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > >     VirtIONet *n = VIRTIO_NET(dev);
> > >     NetClientState *nc;
> > >     int i;
> > >
> > >     if (n->net_conf.mtu) {
> > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > >     }
> > >
> > > The above code can be interpreted as follows:
> > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> > device (that actual value is ignored).
> > >
> > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that
> > we should read the actual limitation for the device.
> > >
> > > If this makes sense I can send a patch to fix this.
> > 
> > Well it will then either have to be for vdpa only, or have
> > compat machinery to avoid breaking migration.
> > 
> 
> How about this one:
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 1067e72b3975..e464e4645c79 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -3188,6 +3188,7 @@ static void virtio_net_guest_notifier_mask(VirtIODevice *vdev, int idx,
>  static void virtio_net_set_config_size(VirtIONet *n, uint64_t host_features)
>  {
>      virtio_add_feature(&host_features, VIRTIO_NET_F_MAC);
> +    virtio_add_feature(&host_features, VIRTIO_NET_F_MTU);
> 
>      n->config_size = virtio_feature_get_config_size(feature_sizes,
>                                                      host_features);

Seems to increase config size unconditionally?

> @@ -3512,6 +3513,7 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> 
>     if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
>          struct virtio_net_config netcfg = {};
> +        n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
>          memcpy(&netcfg.mac, &n->nic_conf.macaddr, ETH_ALEN);
>          vhost_net_set_config(get_vhost_net(nc->peer),
>              (uint8_t *)&netcfg, 0, ETH_ALEN, VHOST_SET_CONFIG_TYPE_MASTER);

And the point is vdpa does not support migration anyway ATM, right?

-- 
MST

RE: VIRTIO_NET_F_MTU not negotiated

[Eli Cohen]

> -----Original Message-----
> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Wednesday, July 27, 2022 12:35 PM
> To: Eli Cohen <elic@nvidia.com>
> Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> virtualization@lists.linux-foundation.org
> Subject: Re: VIRTIO_NET_F_MTU not negotiated
> 
> On Wed, Jul 27, 2022 at 09:04:47AM +0000, Eli Cohen wrote:
> > > -----Original Message-----
> > > From: Michael S. Tsirkin <mst@redhat.com>
> > > Sent: Wednesday, July 27, 2022 10:25 AM
> > > To: Eli Cohen <elic@nvidia.com>
> > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > > virtualization@lists.linux-foundation.org
> > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > >
> > > On Wed, Jul 27, 2022 at 06:51:56AM +0000, Eli Cohen wrote:
> > > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net
> device
> > > (e.g. through libvirt <mtu size="9000"/>).
> > > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom
> to do
> > > it using its copy of virtio_net_config.
> > > >
> > > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > > >
> > > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > {
> > > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > >     VirtIONet *n = VIRTIO_NET(dev);
> > > >     NetClientState *nc;
> > > >     int i;
> > > >
> > > >     if (n->net_conf.mtu) {
> > > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > >     }
> > > >
> > > > The above code can be interpreted as follows:
> > > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> > > device (that actual value is ignored).
> > > >
> > > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates
> that
> > > we should read the actual limitation for the device.
> > > >
> > > > If this makes sense I can send a patch to fix this.
> > >
> > > Well it will then either have to be for vdpa only, or have
> > > compat machinery to avoid breaking migration.
> > >
> >
> > How about this one:
> >
> > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > index 1067e72b3975..e464e4645c79 100644
> > --- a/hw/net/virtio-net.c
> > +++ b/hw/net/virtio-net.c
> > @@ -3188,6 +3188,7 @@ static void virtio_net_guest_notifier_mask(VirtIODevice *vdev, int idx,
> >  static void virtio_net_set_config_size(VirtIONet *n, uint64_t host_features)
> >  {
> >      virtio_add_feature(&host_features, VIRTIO_NET_F_MAC);
> > +    virtio_add_feature(&host_features, VIRTIO_NET_F_MTU);
> >
> >      n->config_size = virtio_feature_get_config_size(feature_sizes,
> >                                                      host_features);
> 
> Seems to increase config size unconditionally?

Right but you pay for reading two more bytes. Is that such a high price to pay?

> 
> > @@ -3512,6 +3513,7 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> >
> >     if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
> >          struct virtio_net_config netcfg = {};
> > +        n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> >          memcpy(&netcfg.mac, &n->nic_conf.macaddr, ETH_ALEN);
> >          vhost_net_set_config(get_vhost_net(nc->peer),
> >              (uint8_t *)&netcfg, 0, ETH_ALEN, VHOST_SET_CONFIG_TYPE_MASTER);
> 
> And the point is vdpa does not support migration anyway ATM, right?
> 

I don't see how this can affect vdpa live migration. Am I missing something?

Re: VIRTIO_NET_F_MTU not negotiated

[Michael S. Tsirkin]

On Wed, Jul 27, 2022 at 10:16:19AM +0000, Eli Cohen wrote:
> > -----Original Message-----
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Wednesday, July 27, 2022 12:35 PM
> > To: Eli Cohen <elic@nvidia.com>
> > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > virtualization@lists.linux-foundation.org
> > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > 
> > On Wed, Jul 27, 2022 at 09:04:47AM +0000, Eli Cohen wrote:
> > > > -----Original Message-----
> > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > Sent: Wednesday, July 27, 2022 10:25 AM
> > > > To: Eli Cohen <elic@nvidia.com>
> > > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > > > virtualization@lists.linux-foundation.org
> > > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > > >
> > > > On Wed, Jul 27, 2022 at 06:51:56AM +0000, Eli Cohen wrote:
> > > > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net
> > device
> > > > (e.g. through libvirt <mtu size="9000"/>).
> > > > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom
> > to do
> > > > it using its copy of virtio_net_config.
> > > > >
> > > > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > > > >
> > > > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > > {
> > > > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > > >     VirtIONet *n = VIRTIO_NET(dev);
> > > > >     NetClientState *nc;
> > > > >     int i;
> > > > >
> > > > >     if (n->net_conf.mtu) {
> > > > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > > >     }
> > > > >
> > > > > The above code can be interpreted as follows:
> > > > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> > > > device (that actual value is ignored).
> > > > >
> > > > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates
> > that
> > > > we should read the actual limitation for the device.
> > > > >
> > > > > If this makes sense I can send a patch to fix this.
> > > >
> > > > Well it will then either have to be for vdpa only, or have
> > > > compat machinery to avoid breaking migration.
> > > >
> > >
> > > How about this one:
> > >
> > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > > index 1067e72b3975..e464e4645c79 100644
> > > --- a/hw/net/virtio-net.c
> > > +++ b/hw/net/virtio-net.c
> > > @@ -3188,6 +3188,7 @@ static void virtio_net_guest_notifier_mask(VirtIODevice *vdev, int idx,
> > >  static void virtio_net_set_config_size(VirtIONet *n, uint64_t host_features)
> > >  {
> > >      virtio_add_feature(&host_features, VIRTIO_NET_F_MAC);
> > > +    virtio_add_feature(&host_features, VIRTIO_NET_F_MTU);
> > >
> > >      n->config_size = virtio_feature_get_config_size(feature_sizes,
> > >                                                      host_features);
> > 
> > Seems to increase config size unconditionally?
> 
> Right but you pay for reading two more bytes. Is that such a high price to pay?


That's not a performance question. The issue compatibility, size
should not change for a given machine type.


> > 
> > > @@ -3512,6 +3513,7 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > >
> > >     if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
> > >          struct virtio_net_config netcfg = {};
> > > +        n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > >          memcpy(&netcfg.mac, &n->nic_conf.macaddr, ETH_ALEN);
> > >          vhost_net_set_config(get_vhost_net(nc->peer),
> > >              (uint8_t *)&netcfg, 0, ETH_ALEN, VHOST_SET_CONFIG_TYPE_MASTER);
> > 
> > And the point is vdpa does not support migration anyway ATM, right?
> > 
> 
> I don't see how this can affect vdpa live migration. Am I missing something?

config size affects things like pci BAR size. This must not change
during migration.

-- 
MST

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: VIRTIO_NET_F_MTU not negotiated

[Michael S. Tsirkin]

On Wed, Jul 27, 2022 at 10:16:19AM +0000, Eli Cohen wrote:
> > -----Original Message-----
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Wednesday, July 27, 2022 12:35 PM
> > To: Eli Cohen <elic@nvidia.com>
> > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > virtualization@lists.linux-foundation.org
> > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > 
> > On Wed, Jul 27, 2022 at 09:04:47AM +0000, Eli Cohen wrote:
> > > > -----Original Message-----
> > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > Sent: Wednesday, July 27, 2022 10:25 AM
> > > > To: Eli Cohen <elic@nvidia.com>
> > > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > > > virtualization@lists.linux-foundation.org
> > > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > > >
> > > > On Wed, Jul 27, 2022 at 06:51:56AM +0000, Eli Cohen wrote:
> > > > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net
> > device
> > > > (e.g. through libvirt <mtu size="9000"/>).
> > > > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom
> > to do
> > > > it using its copy of virtio_net_config.
> > > > >
> > > > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > > > >
> > > > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > > {
> > > > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > > >     VirtIONet *n = VIRTIO_NET(dev);
> > > > >     NetClientState *nc;
> > > > >     int i;
> > > > >
> > > > >     if (n->net_conf.mtu) {
> > > > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > > >     }
> > > > >
> > > > > The above code can be interpreted as follows:
> > > > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> > > > device (that actual value is ignored).
> > > > >
> > > > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates
> > that
> > > > we should read the actual limitation for the device.
> > > > >
> > > > > If this makes sense I can send a patch to fix this.
> > > >
> > > > Well it will then either have to be for vdpa only, or have
> > > > compat machinery to avoid breaking migration.
> > > >
> > >
> > > How about this one:
> > >
> > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > > index 1067e72b3975..e464e4645c79 100644
> > > --- a/hw/net/virtio-net.c
> > > +++ b/hw/net/virtio-net.c
> > > @@ -3188,6 +3188,7 @@ static void virtio_net_guest_notifier_mask(VirtIODevice *vdev, int idx,
> > >  static void virtio_net_set_config_size(VirtIONet *n, uint64_t host_features)
> > >  {
> > >      virtio_add_feature(&host_features, VIRTIO_NET_F_MAC);
> > > +    virtio_add_feature(&host_features, VIRTIO_NET_F_MTU);
> > >
> > >      n->config_size = virtio_feature_get_config_size(feature_sizes,
> > >                                                      host_features);
> > 
> > Seems to increase config size unconditionally?
> 
> Right but you pay for reading two more bytes. Is that such a high price to pay?


That's not a performance question. The issue compatibility, size
should not change for a given machine type.


> > 
> > > @@ -3512,6 +3513,7 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > >
> > >     if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
> > >          struct virtio_net_config netcfg = {};
> > > +        n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > >          memcpy(&netcfg.mac, &n->nic_conf.macaddr, ETH_ALEN);
> > >          vhost_net_set_config(get_vhost_net(nc->peer),
> > >              (uint8_t *)&netcfg, 0, ETH_ALEN, VHOST_SET_CONFIG_TYPE_MASTER);
> > 
> > And the point is vdpa does not support migration anyway ATM, right?
> > 
> 
> I don't see how this can affect vdpa live migration. Am I missing something?

config size affects things like pci BAR size. This must not change
during migration.

-- 
MST

RE: VIRTIO_NET_F_MTU not negotiated

[Eli Cohen]

> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Wednesday, July 27, 2022 6:44 PM
> To: Eli Cohen <elic@nvidia.com>
> Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> virtualization@lists.linux-foundation.org
> Subject: Re: VIRTIO_NET_F_MTU not negotiated
> 
> On Wed, Jul 27, 2022 at 10:16:19AM +0000, Eli Cohen wrote:
> > > -----Original Message-----
> > > From: Michael S. Tsirkin <mst@redhat.com>
> > > Sent: Wednesday, July 27, 2022 12:35 PM
> > > To: Eli Cohen <elic@nvidia.com>
> > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > > virtualization@lists.linux-foundation.org
> > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > >
> > > On Wed, Jul 27, 2022 at 09:04:47AM +0000, Eli Cohen wrote:
> > > > > -----Original Message-----
> > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > Sent: Wednesday, July 27, 2022 10:25 AM
> > > > > To: Eli Cohen <elic@nvidia.com>
> > > > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > > > > virtualization@lists.linux-foundation.org
> > > > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > > > >
> > > > > On Wed, Jul 27, 2022 at 06:51:56AM +0000, Eli Cohen wrote:
> > > > > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net
> > > device
> > > > > (e.g. through libvirt <mtu size="9000"/>).
> > > > > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the
> freedom
> > > to do
> > > > > it using its copy of virtio_net_config.
> > > > > >
> > > > > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > > > > >
> > > > > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > > > {
> > > > > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > > > >     VirtIONet *n = VIRTIO_NET(dev);
> > > > > >     NetClientState *nc;
> > > > > >     int i;
> > > > > >
> > > > > >     if (n->net_conf.mtu) {
> > > > > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > > > >     }
> > > > > >
> > > > > > The above code can be interpreted as follows:
> > > > > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from
> the
> > > > > device (that actual value is ignored).
> > > > > >
> > > > > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates
> > > that
> > > > > we should read the actual limitation for the device.
> > > > > >
> > > > > > If this makes sense I can send a patch to fix this.
> > > > >
> > > > > Well it will then either have to be for vdpa only, or have
> > > > > compat machinery to avoid breaking migration.
> > > > >
> > > >
> > > > How about this one:
> > > >
> > > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > > > index 1067e72b3975..e464e4645c79 100644
> > > > --- a/hw/net/virtio-net.c
> > > > +++ b/hw/net/virtio-net.c
> > > > @@ -3188,6 +3188,7 @@ static void virtio_net_guest_notifier_mask(VirtIODevice *vdev, int idx,
> > > >  static void virtio_net_set_config_size(VirtIONet *n, uint64_t host_features)
> > > >  {
> > > >      virtio_add_feature(&host_features, VIRTIO_NET_F_MAC);
> > > > +    virtio_add_feature(&host_features, VIRTIO_NET_F_MTU);
> > > >
> > > >      n->config_size = virtio_feature_get_config_size(feature_sizes,
> > > >                                                      host_features);
> > >
> > > Seems to increase config size unconditionally?
> >
> > Right but you pay for reading two more bytes. Is that such a high price to pay?
> 
> 
> That's not a performance question. The issue compatibility, size
> should not change for a given machine type.
> 

Did you mean it should not change for virtio_net pci devices?
Can't management controlling the live migration process take care of this?

> 
> > >
> > > > @@ -3512,6 +3513,7 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > >
> > > >     if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
> > > >          struct virtio_net_config netcfg = {};
> > > > +        n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > >          memcpy(&netcfg.mac, &n->nic_conf.macaddr, ETH_ALEN);
> > > >          vhost_net_set_config(get_vhost_net(nc->peer),
> > > >              (uint8_t *)&netcfg, 0, ETH_ALEN, VHOST_SET_CONFIG_TYPE_MASTER);
> > >
> > > And the point is vdpa does not support migration anyway ATM, right?
> > >
> >
> > I don't see how this can affect vdpa live migration. Am I missing something?
> 
> config size affects things like pci BAR size. This must not change
> during migration.
> 

Why should this change during live migration?

> --
> MST

Re: VIRTIO_NET_F_MTU not negotiated

[Michael S. Tsirkin]

On Thu, Jul 28, 2022 at 05:51:32AM +0000, Eli Cohen wrote:
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Wednesday, July 27, 2022 6:44 PM
> > To: Eli Cohen <elic@nvidia.com>
> > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > virtualization@lists.linux-foundation.org
> > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > 
> > On Wed, Jul 27, 2022 at 10:16:19AM +0000, Eli Cohen wrote:
> > > > -----Original Message-----
> > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > Sent: Wednesday, July 27, 2022 12:35 PM
> > > > To: Eli Cohen <elic@nvidia.com>
> > > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > > > virtualization@lists.linux-foundation.org
> > > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > > >
> > > > On Wed, Jul 27, 2022 at 09:04:47AM +0000, Eli Cohen wrote:
> > > > > > -----Original Message-----
> > > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > > Sent: Wednesday, July 27, 2022 10:25 AM
> > > > > > To: Eli Cohen <elic@nvidia.com>
> > > > > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > > > > > virtualization@lists.linux-foundation.org
> > > > > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > > > > >
> > > > > > On Wed, Jul 27, 2022 at 06:51:56AM +0000, Eli Cohen wrote:
> > > > > > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net
> > > > device
> > > > > > (e.g. through libvirt <mtu size="9000"/>).
> > > > > > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the
> > freedom
> > > > to do
> > > > > > it using its copy of virtio_net_config.
> > > > > > >
> > > > > > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > > > > > >
> > > > > > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > > > > {
> > > > > > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > > > > >     VirtIONet *n = VIRTIO_NET(dev);
> > > > > > >     NetClientState *nc;
> > > > > > >     int i;
> > > > > > >
> > > > > > >     if (n->net_conf.mtu) {
> > > > > > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > > > > >     }
> > > > > > >
> > > > > > > The above code can be interpreted as follows:
> > > > > > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from
> > the
> > > > > > device (that actual value is ignored).
> > > > > > >
> > > > > > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates
> > > > that
> > > > > > we should read the actual limitation for the device.
> > > > > > >
> > > > > > > If this makes sense I can send a patch to fix this.
> > > > > >
> > > > > > Well it will then either have to be for vdpa only, or have
> > > > > > compat machinery to avoid breaking migration.
> > > > > >
> > > > >
> > > > > How about this one:
> > > > >
> > > > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > > > > index 1067e72b3975..e464e4645c79 100644
> > > > > --- a/hw/net/virtio-net.c
> > > > > +++ b/hw/net/virtio-net.c
> > > > > @@ -3188,6 +3188,7 @@ static void virtio_net_guest_notifier_mask(VirtIODevice *vdev, int idx,
> > > > >  static void virtio_net_set_config_size(VirtIONet *n, uint64_t host_features)
> > > > >  {
> > > > >      virtio_add_feature(&host_features, VIRTIO_NET_F_MAC);
> > > > > +    virtio_add_feature(&host_features, VIRTIO_NET_F_MTU);
> > > > >
> > > > >      n->config_size = virtio_feature_get_config_size(feature_sizes,
> > > > >                                                      host_features);
> > > >
> > > > Seems to increase config size unconditionally?
> > >
> > > Right but you pay for reading two more bytes. Is that such a high price to pay?
> > 
> > 
> > That's not a performance question. The issue compatibility, size
> > should not change for a given machine type.
> > 
> 
> Did you mean it should not change for virtio_net pci devices?

yes

> Can't management controlling the live migration process take care of this?

Management does what it always did which is set flags consistently.
If we tweak them with virtio_add_feature it can do nothing.

> > 
> > > >
> > > > > @@ -3512,6 +3513,7 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > >
> > > > >     if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
> > > > >          struct virtio_net_config netcfg = {};
> > > > > +        n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > > >          memcpy(&netcfg.mac, &n->nic_conf.macaddr, ETH_ALEN);
> > > > >          vhost_net_set_config(get_vhost_net(nc->peer),
> > > > >              (uint8_t *)&netcfg, 0, ETH_ALEN, VHOST_SET_CONFIG_TYPE_MASTER);
> > > >
> > > > And the point is vdpa does not support migration anyway ATM, right?
> > > >
> > >
> > > I don't see how this can affect vdpa live migration. Am I missing something?
> > 
> > config size affects things like pci BAR size. This must not change
> > during migration.
> > 
> 
> Why should this change during live migration?

Simply put features need to match on both ends.

> > --
> > MST

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: VIRTIO_NET_F_MTU not negotiated

[Michael S. Tsirkin]

On Thu, Jul 28, 2022 at 05:51:32AM +0000, Eli Cohen wrote:
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Wednesday, July 27, 2022 6:44 PM
> > To: Eli Cohen <elic@nvidia.com>
> > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > virtualization@lists.linux-foundation.org
> > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > 
> > On Wed, Jul 27, 2022 at 10:16:19AM +0000, Eli Cohen wrote:
> > > > -----Original Message-----
> > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > Sent: Wednesday, July 27, 2022 12:35 PM
> > > > To: Eli Cohen <elic@nvidia.com>
> > > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > > > virtualization@lists.linux-foundation.org
> > > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > > >
> > > > On Wed, Jul 27, 2022 at 09:04:47AM +0000, Eli Cohen wrote:
> > > > > > -----Original Message-----
> > > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > > Sent: Wednesday, July 27, 2022 10:25 AM
> > > > > > To: Eli Cohen <elic@nvidia.com>
> > > > > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Jason Wang <jasowang@redhat.com>;
> > > > > > virtualization@lists.linux-foundation.org
> > > > > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > > > > >
> > > > > > On Wed, Jul 27, 2022 at 06:51:56AM +0000, Eli Cohen wrote:
> > > > > > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net
> > > > device
> > > > > > (e.g. through libvirt <mtu size="9000"/>).
> > > > > > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the
> > freedom
> > > > to do
> > > > > > it using its copy of virtio_net_config.
> > > > > > >
> > > > > > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > > > > > >
> > > > > > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > > > > {
> > > > > > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > > > > >     VirtIONet *n = VIRTIO_NET(dev);
> > > > > > >     NetClientState *nc;
> > > > > > >     int i;
> > > > > > >
> > > > > > >     if (n->net_conf.mtu) {
> > > > > > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > > > > >     }
> > > > > > >
> > > > > > > The above code can be interpreted as follows:
> > > > > > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from
> > the
> > > > > > device (that actual value is ignored).
> > > > > > >
> > > > > > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates
> > > > that
> > > > > > we should read the actual limitation for the device.
> > > > > > >
> > > > > > > If this makes sense I can send a patch to fix this.
> > > > > >
> > > > > > Well it will then either have to be for vdpa only, or have
> > > > > > compat machinery to avoid breaking migration.
> > > > > >
> > > > >
> > > > > How about this one:
> > > > >
> > > > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > > > > index 1067e72b3975..e464e4645c79 100644
> > > > > --- a/hw/net/virtio-net.c
> > > > > +++ b/hw/net/virtio-net.c
> > > > > @@ -3188,6 +3188,7 @@ static void virtio_net_guest_notifier_mask(VirtIODevice *vdev, int idx,
> > > > >  static void virtio_net_set_config_size(VirtIONet *n, uint64_t host_features)
> > > > >  {
> > > > >      virtio_add_feature(&host_features, VIRTIO_NET_F_MAC);
> > > > > +    virtio_add_feature(&host_features, VIRTIO_NET_F_MTU);
> > > > >
> > > > >      n->config_size = virtio_feature_get_config_size(feature_sizes,
> > > > >                                                      host_features);
> > > >
> > > > Seems to increase config size unconditionally?
> > >
> > > Right but you pay for reading two more bytes. Is that such a high price to pay?
> > 
> > 
> > That's not a performance question. The issue compatibility, size
> > should not change for a given machine type.
> > 
> 
> Did you mean it should not change for virtio_net pci devices?

yes

> Can't management controlling the live migration process take care of this?

Management does what it always did which is set flags consistently.
If we tweak them with virtio_add_feature it can do nothing.

> > 
> > > >
> > > > > @@ -3512,6 +3513,7 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > >
> > > > >     if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
> > > > >          struct virtio_net_config netcfg = {};
> > > > > +        n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > > >          memcpy(&netcfg.mac, &n->nic_conf.macaddr, ETH_ALEN);
> > > > >          vhost_net_set_config(get_vhost_net(nc->peer),
> > > > >              (uint8_t *)&netcfg, 0, ETH_ALEN, VHOST_SET_CONFIG_TYPE_MASTER);
> > > >
> > > > And the point is vdpa does not support migration anyway ATM, right?
> > > >
> > >
> > > I don't see how this can affect vdpa live migration. Am I missing something?
> > 
> > config size affects things like pci BAR size. This must not change
> > during migration.
> > 
> 
> Why should this change during live migration?

Simply put features need to match on both ends.

> > --
> > MST

Re: VIRTIO_NET_F_MTU not negotiated

[Jason Wang]

On Wed, Jul 27, 2022 at 2:52 PM Eli Cohen <elic@nvidia.com> wrote:
>
> I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device (e.g. through libvirt <mtu size="9000"/>).
> Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do it using its copy of virtio_net_config.
>
> The code in qemu that is responsible to allow to consider the device MTU restriction is here:
>
> static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> {
>     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>     VirtIONet *n = VIRTIO_NET(dev);
>     NetClientState *nc;
>     int i;
>
>     if (n->net_conf.mtu) {
>         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
>     }
>
> The above code can be interpreted as follows:
> if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the device (that actual value is ignored).
>
> I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that we should read the actual limitation for the device.
>
> If this makes sense I can send a patch to fix this.

I wonder whether it's worth to bother:

1) mgmt (above libvirt) should have the knowledge to prepare the correct XML
2) it's not specific to MTU, we had other features work like, for
example, the multiqueue?

Thanks

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: VIRTIO_NET_F_MTU not negotiated

[Jason Wang]

On Wed, Jul 27, 2022 at 2:52 PM Eli Cohen <elic@nvidia.com> wrote:
>
> I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device (e.g. through libvirt <mtu size="9000"/>).
> Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do it using its copy of virtio_net_config.
>
> The code in qemu that is responsible to allow to consider the device MTU restriction is here:
>
> static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> {
>     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>     VirtIONet *n = VIRTIO_NET(dev);
>     NetClientState *nc;
>     int i;
>
>     if (n->net_conf.mtu) {
>         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
>     }
>
> The above code can be interpreted as follows:
> if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the device (that actual value is ignored).
>
> I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that we should read the actual limitation for the device.
>
> If this makes sense I can send a patch to fix this.

I wonder whether it's worth to bother:

1) mgmt (above libvirt) should have the knowledge to prepare the correct XML
2) it's not specific to MTU, we had other features work like, for
example, the multiqueue?

Thanks

RE: VIRTIO_NET_F_MTU not negotiated

[Eli Cohen]

> From: Jason Wang <jasowang@redhat.com>
> Sent: Thursday, July 28, 2022 5:09 AM
> To: Eli Cohen <elic@nvidia.com>
> Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Michael S. Tsirkin <mst@redhat.com>;
> virtualization@lists.linux-foundation.org
> Subject: Re: VIRTIO_NET_F_MTU not negotiated
> 
> On Wed, Jul 27, 2022 at 2:52 PM Eli Cohen <elic@nvidia.com> wrote:
> >
> > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device
> (e.g. through libvirt <mtu size="9000"/>).
> > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do
> it using its copy of virtio_net_config.
> >
> > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> >
> > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > {
> >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> >     VirtIONet *n = VIRTIO_NET(dev);
> >     NetClientState *nc;
> >     int i;
> >
> >     if (n->net_conf.mtu) {
> >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> >     }
> >
> > The above code can be interpreted as follows:
> > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> device (that actual value is ignored).
> >
> > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that
> we should read the actual limitation for the device.
> >
> > If this makes sense I can send a patch to fix this.
> 
> I wonder whether it's worth to bother:
> 
> 1) mgmt (above libvirt) should have the knowledge to prepare the correct XML
> 2) it's not specific to MTU, we had other features work like, for
> example, the multiqueue?
> 


Currently libvirt does not recognize setting the mtu through XML for vdpa device. So you mean the fix should go to libvirt?
Furthermore, even if libvirt supports MTU configuration for a vdpa device, the actual value provided will be ignored and the limitation will be taken from what the vdpa device published in its virtio_net_config structure. That makes the XML configuration binary.

> Thanks

Re: VIRTIO_NET_F_MTU not negotiated

[Jason Wang]

On Thu, Jul 28, 2022 at 1:39 PM Eli Cohen <elic@nvidia.com> wrote:
>
> > From: Jason Wang <jasowang@redhat.com>
> > Sent: Thursday, July 28, 2022 5:09 AM
> > To: Eli Cohen <elic@nvidia.com>
> > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Michael S. Tsirkin <mst@redhat.com>;
> > virtualization@lists.linux-foundation.org
> > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> >
> > On Wed, Jul 27, 2022 at 2:52 PM Eli Cohen <elic@nvidia.com> wrote:
> > >
> > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device
> > (e.g. through libvirt <mtu size="9000"/>).
> > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do
> > it using its copy of virtio_net_config.
> > >
> > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > >
> > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > {
> > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > >     VirtIONet *n = VIRTIO_NET(dev);
> > >     NetClientState *nc;
> > >     int i;
> > >
> > >     if (n->net_conf.mtu) {
> > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > >     }
> > >
> > > The above code can be interpreted as follows:
> > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> > device (that actual value is ignored).
> > >
> > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that
> > we should read the actual limitation for the device.
> > >
> > > If this makes sense I can send a patch to fix this.
> >
> > I wonder whether it's worth to bother:
> >
> > 1) mgmt (above libvirt) should have the knowledge to prepare the correct XML
> > 2) it's not specific to MTU, we had other features work like, for
> > example, the multiqueue?
> >
>
>
> Currently libvirt does not recognize setting the mtu through XML for vdpa device. So you mean the fix should go to libvirt?

Probably.

> Furthermore, even if libvirt supports MTU configuration for a vdpa device, the actual value provided will be ignored and the limitation will be taken from what the vdpa device published in its virtio_net_config structure. That makes the XML configuration binary.

Yes, we suffer from a similar issue for "queues=". I think we should
fix qemu by failing the initialization if the value provided by cli
doesn't match what is read from config space.

E.g when mtu=9000 was set by cli but the actual mtu is 1500.

Thanks

>
> > Thanks
>

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: VIRTIO_NET_F_MTU not negotiated

[Jason Wang]

On Thu, Jul 28, 2022 at 1:39 PM Eli Cohen <elic@nvidia.com> wrote:
>
> > From: Jason Wang <jasowang@redhat.com>
> > Sent: Thursday, July 28, 2022 5:09 AM
> > To: Eli Cohen <elic@nvidia.com>
> > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Michael S. Tsirkin <mst@redhat.com>;
> > virtualization@lists.linux-foundation.org
> > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> >
> > On Wed, Jul 27, 2022 at 2:52 PM Eli Cohen <elic@nvidia.com> wrote:
> > >
> > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device
> > (e.g. through libvirt <mtu size="9000"/>).
> > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do
> > it using its copy of virtio_net_config.
> > >
> > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > >
> > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > {
> > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > >     VirtIONet *n = VIRTIO_NET(dev);
> > >     NetClientState *nc;
> > >     int i;
> > >
> > >     if (n->net_conf.mtu) {
> > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > >     }
> > >
> > > The above code can be interpreted as follows:
> > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> > device (that actual value is ignored).
> > >
> > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that
> > we should read the actual limitation for the device.
> > >
> > > If this makes sense I can send a patch to fix this.
> >
> > I wonder whether it's worth to bother:
> >
> > 1) mgmt (above libvirt) should have the knowledge to prepare the correct XML
> > 2) it's not specific to MTU, we had other features work like, for
> > example, the multiqueue?
> >
>
>
> Currently libvirt does not recognize setting the mtu through XML for vdpa device. So you mean the fix should go to libvirt?

Probably.

> Furthermore, even if libvirt supports MTU configuration for a vdpa device, the actual value provided will be ignored and the limitation will be taken from what the vdpa device published in its virtio_net_config structure. That makes the XML configuration binary.

Yes, we suffer from a similar issue for "queues=". I think we should
fix qemu by failing the initialization if the value provided by cli
doesn't match what is read from config space.

E.g when mtu=9000 was set by cli but the actual mtu is 1500.

Thanks

>
> > Thanks
>

Re: VIRTIO_NET_F_MTU not negotiated

[Michael S. Tsirkin]

On Thu, Jul 28, 2022 at 01:51:37PM +0800, Jason Wang wrote:
> On Thu, Jul 28, 2022 at 1:39 PM Eli Cohen <elic@nvidia.com> wrote:
> >
> > > From: Jason Wang <jasowang@redhat.com>
> > > Sent: Thursday, July 28, 2022 5:09 AM
> > > To: Eli Cohen <elic@nvidia.com>
> > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Michael S. Tsirkin <mst@redhat.com>;
> > > virtualization@lists.linux-foundation.org
> > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > >
> > > On Wed, Jul 27, 2022 at 2:52 PM Eli Cohen <elic@nvidia.com> wrote:
> > > >
> > > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device
> > > (e.g. through libvirt <mtu size="9000"/>).
> > > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do
> > > it using its copy of virtio_net_config.
> > > >
> > > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > > >
> > > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > {
> > > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > >     VirtIONet *n = VIRTIO_NET(dev);
> > > >     NetClientState *nc;
> > > >     int i;
> > > >
> > > >     if (n->net_conf.mtu) {
> > > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > >     }
> > > >
> > > > The above code can be interpreted as follows:
> > > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> > > device (that actual value is ignored).
> > > >
> > > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that
> > > we should read the actual limitation for the device.
> > > >
> > > > If this makes sense I can send a patch to fix this.
> > >
> > > I wonder whether it's worth to bother:
> > >
> > > 1) mgmt (above libvirt) should have the knowledge to prepare the correct XML
> > > 2) it's not specific to MTU, we had other features work like, for
> > > example, the multiqueue?
> > >
> >
> >
> > Currently libvirt does not recognize setting the mtu through XML for vdpa device. So you mean the fix should go to libvirt?
> 
> Probably.
> 
> > Furthermore, even if libvirt supports MTU configuration for a vdpa device, the actual value provided will be ignored and the limitation will be taken from what the vdpa device published in its virtio_net_config structure. That makes the XML configuration binary.
> 
> Yes, we suffer from a similar issue for "queues=". I think we should
> fix qemu by failing the initialization if the value provided by cli
> doesn't match what is read from config space.
> 
> E.g when mtu=9000 was set by cli but the actual mtu is 1500.
> 
> Thanks


Jason most features are passthrough now, no?
Why do you want to make this one special?



> >
> > > Thanks
> >

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: VIRTIO_NET_F_MTU not negotiated

[Michael S. Tsirkin]

On Thu, Jul 28, 2022 at 01:51:37PM +0800, Jason Wang wrote:
> On Thu, Jul 28, 2022 at 1:39 PM Eli Cohen <elic@nvidia.com> wrote:
> >
> > > From: Jason Wang <jasowang@redhat.com>
> > > Sent: Thursday, July 28, 2022 5:09 AM
> > > To: Eli Cohen <elic@nvidia.com>
> > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Michael S. Tsirkin <mst@redhat.com>;
> > > virtualization@lists.linux-foundation.org
> > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > >
> > > On Wed, Jul 27, 2022 at 2:52 PM Eli Cohen <elic@nvidia.com> wrote:
> > > >
> > > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device
> > > (e.g. through libvirt <mtu size="9000"/>).
> > > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do
> > > it using its copy of virtio_net_config.
> > > >
> > > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > > >
> > > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > {
> > > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > >     VirtIONet *n = VIRTIO_NET(dev);
> > > >     NetClientState *nc;
> > > >     int i;
> > > >
> > > >     if (n->net_conf.mtu) {
> > > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > >     }
> > > >
> > > > The above code can be interpreted as follows:
> > > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> > > device (that actual value is ignored).
> > > >
> > > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that
> > > we should read the actual limitation for the device.
> > > >
> > > > If this makes sense I can send a patch to fix this.
> > >
> > > I wonder whether it's worth to bother:
> > >
> > > 1) mgmt (above libvirt) should have the knowledge to prepare the correct XML
> > > 2) it's not specific to MTU, we had other features work like, for
> > > example, the multiqueue?
> > >
> >
> >
> > Currently libvirt does not recognize setting the mtu through XML for vdpa device. So you mean the fix should go to libvirt?
> 
> Probably.
> 
> > Furthermore, even if libvirt supports MTU configuration for a vdpa device, the actual value provided will be ignored and the limitation will be taken from what the vdpa device published in its virtio_net_config structure. That makes the XML configuration binary.
> 
> Yes, we suffer from a similar issue for "queues=". I think we should
> fix qemu by failing the initialization if the value provided by cli
> doesn't match what is read from config space.
> 
> E.g when mtu=9000 was set by cli but the actual mtu is 1500.
> 
> Thanks


Jason most features are passthrough now, no?
Why do you want to make this one special?



> >
> > > Thanks
> >

Re: VIRTIO_NET_F_MTU not negotiated

[Jason Wang]

On Thu, Jul 28, 2022 at 2:48 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Thu, Jul 28, 2022 at 01:51:37PM +0800, Jason Wang wrote:
> > On Thu, Jul 28, 2022 at 1:39 PM Eli Cohen <elic@nvidia.com> wrote:
> > >
> > > > From: Jason Wang <jasowang@redhat.com>
> > > > Sent: Thursday, July 28, 2022 5:09 AM
> > > > To: Eli Cohen <elic@nvidia.com>
> > > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Michael S. Tsirkin <mst@redhat.com>;
> > > > virtualization@lists.linux-foundation.org
> > > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > > >
> > > > On Wed, Jul 27, 2022 at 2:52 PM Eli Cohen <elic@nvidia.com> wrote:
> > > > >
> > > > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device
> > > > (e.g. through libvirt <mtu size="9000"/>).
> > > > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do
> > > > it using its copy of virtio_net_config.
> > > > >
> > > > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > > > >
> > > > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > > {
> > > > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > > >     VirtIONet *n = VIRTIO_NET(dev);
> > > > >     NetClientState *nc;
> > > > >     int i;
> > > > >
> > > > >     if (n->net_conf.mtu) {
> > > > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > > >     }
> > > > >
> > > > > The above code can be interpreted as follows:
> > > > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> > > > device (that actual value is ignored).
> > > > >
> > > > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that
> > > > we should read the actual limitation for the device.
> > > > >
> > > > > If this makes sense I can send a patch to fix this.
> > > >
> > > > I wonder whether it's worth to bother:
> > > >
> > > > 1) mgmt (above libvirt) should have the knowledge to prepare the correct XML
> > > > 2) it's not specific to MTU, we had other features work like, for
> > > > example, the multiqueue?
> > > >
> > >
> > >
> > > Currently libvirt does not recognize setting the mtu through XML for vdpa device. So you mean the fix should go to libvirt?
> >
> > Probably.
> >
> > > Furthermore, even if libvirt supports MTU configuration for a vdpa device, the actual value provided will be ignored and the limitation will be taken from what the vdpa device published in its virtio_net_config structure. That makes the XML configuration binary.
> >
> > Yes, we suffer from a similar issue for "queues=". I think we should
> > fix qemu by failing the initialization if the value provided by cli
> > doesn't match what is read from config space.
> >
> > E.g when mtu=9000 was set by cli but the actual mtu is 1500.
> >
> > Thanks
>
>
> Jason most features are passthrough now, no?
> Why do you want to make this one special?

I don't want to make anything special, but I couldn't find a better approach.

MTU is not the only thing. It applies to all the other features whose
default value is false (MQ, RSS, etc).

Thanks

>
>
>
> > >
> > > > Thanks
> > >
>

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

Re: VIRTIO_NET_F_MTU not negotiated

[Jason Wang]

On Thu, Jul 28, 2022 at 2:48 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Thu, Jul 28, 2022 at 01:51:37PM +0800, Jason Wang wrote:
> > On Thu, Jul 28, 2022 at 1:39 PM Eli Cohen <elic@nvidia.com> wrote:
> > >
> > > > From: Jason Wang <jasowang@redhat.com>
> > > > Sent: Thursday, July 28, 2022 5:09 AM
> > > > To: Eli Cohen <elic@nvidia.com>
> > > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Michael S. Tsirkin <mst@redhat.com>;
> > > > virtualization@lists.linux-foundation.org
> > > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > > >
> > > > On Wed, Jul 27, 2022 at 2:52 PM Eli Cohen <elic@nvidia.com> wrote:
> > > > >
> > > > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device
> > > > (e.g. through libvirt <mtu size="9000"/>).
> > > > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do
> > > > it using its copy of virtio_net_config.
> > > > >
> > > > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > > > >
> > > > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > > {
> > > > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > > >     VirtIONet *n = VIRTIO_NET(dev);
> > > > >     NetClientState *nc;
> > > > >     int i;
> > > > >
> > > > >     if (n->net_conf.mtu) {
> > > > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > > >     }
> > > > >
> > > > > The above code can be interpreted as follows:
> > > > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> > > > device (that actual value is ignored).
> > > > >
> > > > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that
> > > > we should read the actual limitation for the device.
> > > > >
> > > > > If this makes sense I can send a patch to fix this.
> > > >
> > > > I wonder whether it's worth to bother:
> > > >
> > > > 1) mgmt (above libvirt) should have the knowledge to prepare the correct XML
> > > > 2) it's not specific to MTU, we had other features work like, for
> > > > example, the multiqueue?
> > > >
> > >
> > >
> > > Currently libvirt does not recognize setting the mtu through XML for vdpa device. So you mean the fix should go to libvirt?
> >
> > Probably.
> >
> > > Furthermore, even if libvirt supports MTU configuration for a vdpa device, the actual value provided will be ignored and the limitation will be taken from what the vdpa device published in its virtio_net_config structure. That makes the XML configuration binary.
> >
> > Yes, we suffer from a similar issue for "queues=". I think we should
> > fix qemu by failing the initialization if the value provided by cli
> > doesn't match what is read from config space.
> >
> > E.g when mtu=9000 was set by cli but the actual mtu is 1500.
> >
> > Thanks
>
>
> Jason most features are passthrough now, no?
> Why do you want to make this one special?

I don't want to make anything special, but I couldn't find a better approach.

MTU is not the only thing. It applies to all the other features whose
default value is false (MQ, RSS, etc).

Thanks

>
>
>
> > >
> > > > Thanks
> > >
>

Re: VIRTIO_NET_F_MTU not negotiated

[Eugenio Perez Martin]

On Thu, Jul 28, 2022 at 7:51 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Thu, Jul 28, 2022 at 1:39 PM Eli Cohen <elic@nvidia.com> wrote:
> >
> > > From: Jason Wang <jasowang@redhat.com>
> > > Sent: Thursday, July 28, 2022 5:09 AM
> > > To: Eli Cohen <elic@nvidia.com>
> > > Cc: Eugenio Perez Martin <eperezma@redhat.com>; qemu-devel@nongnu.org; Michael S. Tsirkin <mst@redhat.com>;
> > > virtualization@lists.linux-foundation.org
> > > Subject: Re: VIRTIO_NET_F_MTU not negotiated
> > >
> > > On Wed, Jul 27, 2022 at 2:52 PM Eli Cohen <elic@nvidia.com> wrote:
> > > >
> > > > I found out that the reason why I could not enforce the mtu stems from the fact that I did not configure max mtu for the net device
> > > (e.g. through libvirt <mtu size="9000"/>).
> > > > Libvirt does not allow this configuration for vdpa devices and probably for a reason. The vdpa backend driver has the freedom to do
> > > it using its copy of virtio_net_config.
> > > >
> > > > The code in qemu that is responsible to allow to consider the device MTU restriction is here:
> > > >
> > > > static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > {
> > > >     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > >     VirtIONet *n = VIRTIO_NET(dev);
> > > >     NetClientState *nc;
> > > >     int i;
> > > >
> > > >     if (n->net_conf.mtu) {
> > > >         n->host_features |= (1ULL << VIRTIO_NET_F_MTU);
> > > >     }
> > > >
> > > > The above code can be interpreted as follows:
> > > > if the command line arguments of qemu indicates that mtu should be limited, then we would read this mtu limitation from the
> > > device (that actual value is ignored).
> > > >
> > > > I worked around this limitation by unconditionally setting VIRTIO_NET_F_MTU in the host features. As said, it only indicates that
> > > we should read the actual limitation for the device.
> > > >
> > > > If this makes sense I can send a patch to fix this.
> > >
> > > I wonder whether it's worth to bother:
> > >
> > > 1) mgmt (above libvirt) should have the knowledge to prepare the correct XML
> > > 2) it's not specific to MTU, we had other features work like, for
> > > example, the multiqueue?
> > >
> >
> >
> > Currently libvirt does not recognize setting the mtu through XML for vdpa device. So you mean the fix should go to libvirt?
>
> Probably.
>
> > Furthermore, even if libvirt supports MTU configuration for a vdpa device, the actual value provided will be ignored and the limitation will be taken from what the vdpa device published in its virtio_net_config structure. That makes the XML configuration binary.
>
> Yes, we suffer from a similar issue for "queues=". I think we should
> fix qemu by failing the initialization if the value provided by cli
> doesn't match what is read from config space.
>
> E.g when mtu=9000 was set by cli but the actual mtu is 1500.
>

Maybe we can be less strict? Since config space mtu is the maximum
value accepted by the device.

So the case of setting mtu=9000 by cli while the device mtu is 1500
should fail, but not the reverse one. To set a cli mtu <= device mtu
should work in my opinion.

If libvirt has the knowledge to set max mtu properly, it can do so. If
not (or not using libvirt), it should be possible to pass the device's
one.

Other features may need to do the same.

Thanks!