netdev - Re: [iproute2-next v2 4/4] vdpa: Enable user to set mtu of the vdpa device

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Message-ID: <20211220070136-mutt-send-email-mst@kernel.org>
Date:   Mon, 20 Dec 2021 07:02:58 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Parav Pandit <parav@...dia.com>
Cc:     David Ahern <dsahern@...il.com>,
        "stephen@...workplumber.org" <stephen@...workplumber.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "jasowang@...hat.com" <jasowang@...hat.com>
Subject: Re: [iproute2-next v2 4/4] vdpa: Enable user to set mtu of the vdpa
 device

On Mon, Dec 20, 2021 at 03:49:21AM +0000, Parav Pandit wrote:
> 
> 
> > From: Michael S. Tsirkin <mst@...hat.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

After consideration, this future proofing seems like a good thing to have.

-- 
MST