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From: "Michael S. Tsirkin" <mst@redhat.com>
To: Parav Pandit <parav@nvidia.com>
Cc: "stephen@networkplumber.org" <stephen@networkplumber.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"virtualization@lists.linux-foundation.org"
	<virtualization@lists.linux-foundation.org>,
	David Ahern <dsahern@gmail.com>
Subject: Re: [iproute2-next v2 4/4] vdpa: Enable user to set mtu of the vdpa device
Date: Mon, 20 Dec 2021 07:02:58 -0500	[thread overview]
Message-ID: <20211220070136-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <PH0PR12MB548189EBA8346A960A0A409FDC7B9@PH0PR12MB5481.namprd12.prod.outlook.com>

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

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

-- 
MST

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

WARNING: multiple messages have this Message-ID (diff)
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Parav Pandit <parav@nvidia.com>
Cc: David Ahern <dsahern@gmail.com>,
	"stephen@networkplumber.org" <stephen@networkplumber.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"virtualization@lists.linux-foundation.org" 
	<virtualization@lists.linux-foundation.org>,
	"jasowang@redhat.com" <jasowang@redhat.com>
Subject: Re: [iproute2-next v2 4/4] vdpa: Enable user to set mtu of the vdpa device
Date: Mon, 20 Dec 2021 07:02:58 -0500	[thread overview]
Message-ID: <20211220070136-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <PH0PR12MB548189EBA8346A960A0A409FDC7B9@PH0PR12MB5481.namprd12.prod.outlook.com>

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

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

-- 
MST


  reply	other threads:[~2021-12-20 12:03 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-12-17  8:08 [iproute2-next v2 0/4] vdpa tool to query and set config layout Parav Pandit via Virtualization
2021-12-17  8:08 ` Parav Pandit
2021-12-17  8:08 ` [iproute2-next v2 1/4] vdpa: Update kernel headers Parav Pandit via Virtualization
2021-12-17  8:08   ` Parav Pandit
2021-12-17  8:08 ` [iproute2-next v2 2/4] vdpa: Enable user to query vdpa device config layout Parav Pandit via Virtualization
2021-12-17  8:08   ` Parav Pandit
2021-12-17  8:08 ` [iproute2-next v2 3/4] vdpa: Enable user to set mac address of vdpa device Parav Pandit via Virtualization
2021-12-17  8:08   ` Parav Pandit
2021-12-17  8:08 ` [iproute2-next v2 4/4] vdpa: Enable user to set mtu of the " Parav Pandit via Virtualization
2021-12-17  8:08   ` Parav Pandit
2021-12-18 20:53   ` David Ahern
2021-12-18 20:53     ` David Ahern
2021-12-18 22:07     ` Michael S. Tsirkin
2021-12-18 22:07       ` Michael S. Tsirkin
2021-12-20  3:49       ` Parav Pandit via Virtualization
2021-12-20  3:49         ` Parav Pandit
2021-12-20 12:02         ` Michael S. Tsirkin [this message]
2021-12-20 12:02           ` Michael S. Tsirkin
2021-12-20 19:11           ` Parav Pandit via Virtualization
2021-12-20 19:11             ` Parav Pandit
2021-12-20 21:11             ` David Ahern
2021-12-20 21:11               ` David Ahern
2021-12-20 23:06               ` Michael S. Tsirkin
2021-12-20 23:06                 ` Michael S. Tsirkin

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