Re: [RFC v4 0/5] Add packed virtqueue to shadow virtqueue

[Sahil Siddiq]

Hi,

Thank you for your reply.

On 12/10/24 2:57 PM, Eugenio Perez Martin wrote:
> On Thu, Dec 5, 2024 at 9:34 PM Sahil Siddiq <icegambit91@gmail.com> wrote:
> > Hi,
> >
> > There are two issues that I found while trying to test
> > my changes. I thought I would send the patch series
> > as well in case that helps in troubleshooting. I haven't
> > been able to find an issue in the implementation yet.
> > Maybe I am missing something.
> >
> > I have been following the "Hands on vDPA: what do you do
> > when you ain't got the hardware v2 (Part 2)" [1] blog to
> > test my changes. To boot the L1 VM, I ran:
> >
> > [...]
> >
> > But if I boot L2 with x-svq=true as shown below, I am unable
> > to ping the host machine.
> >
> > $ ./qemu/build/qemu-system-x86_64 \
> > -nographic \
> > -m 4G \
> > -enable-kvm \
> > -M q35 \
> > -drive file=//root/L2.qcow2,media=disk,if=virtio \
> > -netdev type=vhost-vdpa,vhostdev=/dev/vhost-vdpa-0,x-svq=true,id=vhost-vdpa0 \
> > -device 
> > virtio-net-pci,netdev=vhost-vdpa0,disable-legacy=on,disable-modern=off,ctrl_vq=on,ctrl_rx=on,event_idx=off,bus=pcie.0,addr=0x7
> >  \
> > -smp 4 \
> > -cpu host \
> > 2>&1 | tee vm.log
> >
> > In L2:
> >
> > # ip addr add 111.1.1.2/24 dev eth0
> > # ip addr show eth0
> > 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP 
> > group default qlen 1000
> >      link/ether 52:54:00:12:34:57 brd ff:ff:ff:ff:ff:ff
> >      altname enp0s7
> >      inet 111.1.1.2/24 scope global eth0
> >         valid_lft forever preferred_lft forever
> >      inet6 fe80::9877:de30:5f17:35f9/64 scope link noprefixroute
> >         valid_lft forever preferred_lft forever
> >
> > # ip route
> > 111.1.1.0/24 dev eth0 proto kernel scope link src 111.1.1.2
> >
> > # ping 111.1.1.1 -w10
> > PING 111.1.1.1 (111.1.1.1) 56(84) bytes of data.
> >  From 111.1.1.2 icmp_seq=1 Destination Host Unreachable
> > ping: sendmsg: No route to host
> >  From 111.1.1.2 icmp_seq=2 Destination Host Unreachable
> >  From 111.1.1.2 icmp_seq=3 Destination Host Unreachable
> >
> > --- 111.1.1.1 ping statistics ---
> > 3 packets transmitted, 0 received, +3 errors, 100% packet loss, time 2076ms
> > pipe 3
> >
> > The other issue is related to booting L2 with "x-svq=true"
> > and "packed=on".
> >
> > In L1:
> >
> > $ ./qemu/build/qemu-system-x86_64 \
> > -nographic \
> > -m 4G \
> > -enable-kvm \
> > -M q35 \
> > -drive file=//root/L2.qcow2,media=disk,if=virtio \
> > -netdev type=vhost-vdpa,vhostdev=/dev/vhost-vdpa-0,id=vhost-vdpa0,x-svq=true \
> > -device 
> > virtio-net-pci,netdev=vhost-vdpa0,disable-legacy=on,disable-modern=off,guest_uso4=off,guest_uso6=off,host_uso=off,guest_announce=off,ctrl_vq=on,ctrl_rx=on,event_idx=off,packed=on,bus=pcie.0,addr=0x7
> >  \
> > -smp 4 \
> > -cpu host \
> > 2>&1 | tee vm.log
> >
> > The kernel throws "virtio_net virtio1: output.0:id 0 is not
> > a head!" [4].
>
> So this series implements the descriptor forwarding from the guest to
> the device in packed vq. We also need to forward the descriptors from
> the device to the guest. The device writes them in the SVQ ring.
>
> The functions responsible for that in QEMU are
> hw/virtio/vhost-shadow-virtqueue.c:vhost_svq_flush, which is called by
> the device when used descriptors are written to the SVQ, which calls
> hw/virtio/vhost-shadow-virtqueue.c:vhost_svq_get_buf. We need to do
> modifications similar to vhost_svq_add: Make them conditional if we're
> in split or packed vq, and "copy" the code from Linux's
> drivers/virtio/virtio_ring.c:virtqueue_get_buf.
>
> After these modifications you should be able to ping and forward
> traffic. As always, It is totally ok if it needs more than one
> iteration, and feel free to ask any question you have :).

Understood, I'll make these changes and will test it again.

Thanks,
Sahil