Re: [PATCH net-next 2/2] tun: AF_XDP Rx zero-copy support

Jason Wang
2024-01-30T02:50:56Z

On Mon, Jan 29, 2024 at 7:40 PM wangyunjian <wangyunjian@huawei.com> wrote:
>
> > -----Original Message-----
> > From: Jason Wang [mailto:jasowang@redhat.com]
> > Sent: Monday, January 29, 2024 11:03 AM
> > To: wangyunjian <wangyunjian@huawei.com>
> > Cc: mst@redhat.com; willemdebruijn.kernel@gmail.com; kuba@kernel.org;
> > davem@davemloft.net; magnus.karlsson@intel.com; netdev@vger.kernel.org;
> > linux-kernel@vger.kernel.org; kvm@vger.kernel.org;
> > virtualization@lists.linux.dev; xudingke <xudingke@huawei.com>
> > Subject: Re: [PATCH net-next 2/2] tun: AF_XDP Rx zero-copy support
> >
> > On Thu, Jan 25, 2024 at 8:54 PM wangyunjian <wangyunjian@huawei.com>
> > wrote:
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: Jason Wang [mailto:jasowang@redhat.com]
> > > > Sent: Thursday, January 25, 2024 12:49 PM
> > > > To: wangyunjian <wangyunjian@huawei.com>
> > > > Cc: mst@redhat.com; willemdebruijn.kernel@gmail.com;
> > > > kuba@kernel.org; davem@davemloft.net; magnus.karlsson@intel.com;
> > > > netdev@vger.kernel.org; linux-kernel@vger.kernel.org;
> > > > kvm@vger.kernel.org; virtualization@lists.linux.dev; xudingke
> > > > <xudingke@huawei.com>
> > > > Subject: Re: [PATCH net-next 2/2] tun: AF_XDP Rx zero-copy support
> > > >
> > > > On Wed, Jan 24, 2024 at 5:38 PM Yunjian Wang
> > > > <wangyunjian@huawei.com>
> > > > wrote:
> > > > >
> > > > > Now the zero-copy feature of AF_XDP socket is supported by some
> > > > > drivers, which can reduce CPU utilization on the xdp program.
> > > > > This patch set allows tun to support AF_XDP Rx zero-copy feature.
> > > > >
> > > > > This patch tries to address this by:
> > > > > - Use peek_len to consume a xsk->desc and get xsk->desc length.
> > > > > - When the tun support AF_XDP Rx zero-copy, the vq's array maybe empty.
> > > > > So add a check for empty vq's array in vhost_net_buf_produce().
> > > > > - add XDP_SETUP_XSK_POOL and ndo_xsk_wakeup callback support
> > > > > - add tun_put_user_desc function to copy the Rx data to VM
> > > >
> > > > Code explains themselves, let's explain why you need to do this.
> > > >
> > > > 1) why you want to use peek_len
> > > > 2) for "vq's array", what does it mean?
> > > > 3) from the view of TUN/TAP tun_put_user_desc() is the TX path, so I
> > > > guess you meant TX zerocopy instead of RX (as I don't see codes for
> > > > RX?)
> > >
> > > OK, I agree and use TX zerocopy instead of RX zerocopy. I meant RX
> > > zerocopy from the view of vhost-net.
> >
> > Ok.
> >
> > >
> > > >
> > > > A big question is how could you handle GSO packets from
> > userspace/guests?
> > >
> > > Now by disabling VM's TSO and csum feature.
> >
> > Btw, how could you do that?
>
> By set network backend-specific options:
> <driver name='vhost'>
> <host csum='off' gso='off' tso4='off' tso6='off' ecn='off' ufo='off' mrg_rxbuf='off'/>
> <guest csum='off' tso4='off' tso6='off' ecn='off' ufo='off'/>
> </driver>

This is the mgmt work, but the problem is what happens if GSO is not
disabled in the guest, or is there a way to:

1) forcing the guest GSO to be off
2) a graceful fallback

Thanks

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