Virtualization - Dec 2021 - [iproute2-next v2 4/4] vdpa: Enable user to set mtu of the vdpa device

On Sat, Dec 18, 2021 at 01:53:01PM -0700, David Ahern
wrote:
> On 12/17/21 1:08 AM, Parav Pandit wrote:
> > @@ -204,6 +217,8 @@ static void vdpa_opts_put(struct nlmsghdr *nlh,
struct vdpa *vdpa)
> >  	if (opts->present & VDPA_OPT_VDEV_MAC)
> >  		mnl_attr_put(nlh, VDPA_ATTR_DEV_NET_CFG_MACADDR,
> >  			     sizeof(opts->mac), opts->mac);
> > +	if (opts->present & VDPA_OPT_VDEV_MTU)
> > +		mnl_attr_put_u16(nlh, VDPA_ATTR_DEV_NET_CFG_MTU, opts->mtu);
> 
> Why limit the MTU to a u16? Eric for example is working on "Big
TCP"
> where IPv6 can work with Jumbograms where mtu can be > 64k.
> 
> https://datatracker.ietf.org/doc/html/rfc2675
Well it's 16 bit at the virtio level, though we can extend that of
course. Making it match for now removes need for validation.
-- 
MST

> From: Michael S. Tsirkin <mst at redhat.com>
> Sent: Sunday, December 19, 2021 3:37 AM
> 
> On Sat, Dec 18, 2021 at 01:53:01PM -0700, David Ahern wrote:
> > On 12/17/21 1:08 AM, Parav Pandit wrote:
> > > @@ -204,6 +217,8 @@ static void vdpa_opts_put(struct nlmsghdr
*nlh,
> struct vdpa *vdpa)
> > >  	if (opts->present & VDPA_OPT_VDEV_MAC)
> > >  		mnl_attr_put(nlh, VDPA_ATTR_DEV_NET_CFG_MACADDR,
> > >  			     sizeof(opts->mac), opts->mac);
> > > +	if (opts->present & VDPA_OPT_VDEV_MTU)
> > > +		mnl_attr_put_u16(nlh, VDPA_ATTR_DEV_NET_CFG_MTU,
> opts->mtu);
> >
> > Why limit the MTU to a u16? Eric for example is working on "Big
TCP"
> > where IPv6 can work with Jumbograms where mtu can be > 64k.
> >
> > https://datatracker.ietf.org/doc/html/rfc2675
> 
> Well it's 16 bit at the virtio level, though we can extend that of
course. Making
> it match for now removes need for validation.
> --
As Michael mentioned virtio specification limits the mtu to 64k-1. Hence 16-bit.
First we need to update the virtio spec to support > 64K mtu.
However, when/if (I don't know when) that happens, we need to make this also
u32.
So may be we can make it u32 now, but still restrict the mtu value to 64k-1 in
kernel, until the spec is updated.

Let me know, if you think that's future proofing is better, I first need to
update the kernel to take nla u32.
> MST